Although this wasn't (as far as I know) causing any performance issues,
it was making the join-order badness report quite noisy, and so I
figured it was worth fixing.
Before:
```
Tuple counts for RegexTreeView::RegExpBackRef::getGroup#dispred#f0820431#ff/2@d3441d0b after 84ms:
1501195 ~3% {2} r1 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'result', Lhs.1 'result'
149 ~0% {5} r2 = JOIN r1 WITH RegexTreeView::RegExpBackRef#class#31aac2a7#ffff ON FIRST 1 OUTPUT Rhs.1, Rhs.2, Rhs.3, Lhs.1 'result', Lhs.0 'this'
149 ~1% {3} r3 = JOIN r2 WITH regex::RegexString::numbered_backreference#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.3 'result', Rhs.3, Lhs.4 'this'
4 ~0% {2} r4 = JOIN r3 WITH RegexTreeView::RegExpGroup::getNumber#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.2 'this', Lhs.0 'result'
1501195 ~3% {2} r5 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.1 'result', Rhs.1 'result'
42526 ~0% {5} r6 = JOIN r5 WITH RegexTreeView::RegExpGroup#31aac2a7#ffff ON FIRST 1 OUTPUT Lhs.1 'this', Lhs.0 'result', Rhs.1, Rhs.2, Rhs.3
22 ~0% {8} r7 = JOIN r6 WITH RegexTreeView::RegExpBackRef#class#31aac2a7#ffff ON FIRST 1 OUTPUT Lhs.2, Lhs.3, Lhs.4, Lhs.1 'result', Lhs.0 'this', Rhs.1, Rhs.2, Rhs.3
0 ~0% {6} r8 = JOIN r7 WITH regex::RegexString::getGroupName#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.5, Lhs.6, Lhs.7, Rhs.3, Lhs.3 'result', Lhs.4 'this'
0 ~0% {2} r9 = JOIN r8 WITH regex::RegexString::named_backreference#dispred#f0820431#ffff ON FIRST 4 OUTPUT Lhs.5 'this', Lhs.4 'result'
4 ~0% {2} r10 = r4 UNION r9
return r10
```
In this case I opted for a classical solution: tying together the
literal and number (or name) part of the backreference in order to
encourage a two-column join.
After:
```
Tuple counts for RegexTreeView::RegExpBackRef::getGroup#dispred#f0820431#ff/2@b0cc4d5n after 0ms:
898 ~1% {3} r1 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff WITH RegexTreeView::RegExpGroup::getNumber#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.0 'result'
4 ~0% {2} r2 = JOIN r1 WITH RegexTreeView::RegExpBackRef::hasLiteralAndNumber#f0820431#fff_120#join_rhs ON FIRST 2 OUTPUT Rhs.2 'this', Lhs.2 'result'
1110 ~0% {5} r3 = JOIN RegexTreeView::RegExpGroup#31aac2a7#ffff WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Lhs.0 'result', Rhs.1
146 ~0% {3} r4 = JOIN r3 WITH regex::RegexString::getGroupName#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.4, Rhs.3, Lhs.3 'result'
0 ~0% {2} r5 = JOIN r4 WITH RegexTreeView::RegExpBackRef::hasLiteralAndName#f0820431#fff_120#join_rhs ON FIRST 2 OUTPUT Rhs.2 'this', Lhs.2 'result'
4 ~0% {2} r6 = r2 UNION r5
return r6
```
This bad join was identified by the join-order-badness report, which
showed that:
py/use-of-input:MRO::flatten_list#f4eaf05f#fff#9c5fe54whnlqffdgu65vhb8uhpg# (order_500000)
calculated a whopping 212,820,108 tuples in order to produce an output of
size 55516, roughly 3833 times more effort than needed.
Here's a snippet of the slowest iteration of that predicate:
```
Tuple counts for MRO::flatten_list#f4eaf05f#fff/3@i1839#0265eb3w after 14ms:
0 ~0% {3} r1 = JOIN MRO::need_flattening#f4eaf05f#f#prev_delta WITH MRO::ConsList#f4eaf05f#fff#reorder_2_0_1#prev ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'list', Rhs.2
0 ~0% {3} r2 = JOIN r1 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'list', Rhs.1 'n'
0 ~0% {3} r3 = JOIN r2 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
0 ~0% {3} r4 = SCAN MRO::ConsList#f4eaf05f#fff#prev_delta OUTPUT In.2 'list', In.0, In.1
0 ~0% {3} r5 = JOIN r4 WITH MRO::need_flattening#f4eaf05f#f#prev ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.0 'list'
0 ~0% {3} r6 = JOIN r5 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1, Lhs.2 'list', Rhs.1 'n'
0 ~0% {3} r7 = JOIN r6 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
0 ~0% {3} r8 = r3 UNION r7
26355 ~2% {3} r9 = SCAN MRO::ConsList#f4eaf05f#fff#prev OUTPUT In.2 'list', In.0, In.1
0 ~0% {3} r10 = JOIN r9 WITH MRO::need_flattening#f4eaf05f#f#prev ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.0 'list'
0 ~0% {3} r11 = JOIN r10 WITH MRO::ClassList::length#f0820431#ff#prev_delta ON FIRST 1 OUTPUT Lhs.1, Lhs.2 'list', Rhs.1 'n'
0 ~0% {3} r12 = JOIN r11 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
...
```
(... and a bunch more lines. The same construction appears several times,
but the join order is the same each time.)
Clearly it would be better to start with whatever is in `need_flattening`,
and then do the other joins. This is what the present fix does (by
unbinding `list` in all but the `needs_flattening` call).
After the fix, the slowest iteration is as follows:
```
Tuple counts for MRO::flatten_list#f4eaf05f#fff/3@i2617#8155ab3w after 9ms:
0 ~0% {2} r1 = SCAN MRO::need_flattening#f4eaf05f#f#prev_delta OUTPUT In.0 'list', In.0 'list'
0 ~0% {3} r2 = JOIN r1 WITH MRO::ConsList#f4eaf05f#fff#reorder_2_0_1#prev ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'list', Rhs.2
0 ~0% {3} r3 = JOIN r2 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'list', Rhs.1 'n'
0 ~0% {3} r4 = JOIN r3 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
1 ~0% {2} r5 = SCAN MRO::need_flattening#f4eaf05f#f#prev OUTPUT In.0 'list', In.0 'list'
0 ~0% {3} r6 = JOIN r5 WITH MRO::ConsList#f4eaf05f#fff#reorder_2_0_1#prev_delta ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'list', Rhs.2
0 ~0% {3} r7 = JOIN r6 WITH MRO::ClassList::length#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'list', Rhs.1 'n'
0 ~0% {3} r8 = JOIN r7 WITH MRO::ClassListList::flatten#dispred#f0820431#ff#prev ON FIRST 1 OUTPUT Lhs.1 'list', Lhs.2 'n', Rhs.1 'result'
...
```
(... and so on. The remainder is 0 tuples all the way.)
In total, we went from
```
40.6s | 7614 | 15ms @ 1839 | MRO::flatten_list#f4eaf05f#fff@0265eb3w
```
to
```
7.8s | 7614 | 11ms @ 2617 | MRO::flatten_list#f4eaf05f#fff@8155ab3w
```
in the program
```python
if not is_safe(path):
return
```
the last node in the `ConditionBlock` is `not is_safe(path)`,
so it would never match "a call to is_safe".
Thus, guards inside `not` would not be part of `GuardNode`
(nor `BarrierGuard`). Now they can.
Fixes a bad join in `list_of_linearization_of_bases_plus_bases`.
Previvously, we joined together `ConsList` and `getBase` before filtering
these out using the recursive call. Now we do the recursion first.
Co-authored-by: yoff <yoff@github.com>
