Python: Fix bad join in DataFlowDispatch

A case of bad magic. Rather than evaluating separately whether a class
has a method of some name, the compiler opted to magick in the fact
that this was done as part of the `findFunctionAccordingToMro`
predicate. Hilarity ensued.

However, _we_ know that magic really isn't needed in this case (the
number of results is bounded by `Class.getAMethod` since methods have
only a single name), so by factoring it out into a helper predicate, we
can help the join-orderer along.

Before
```
(377s) Starting to evaluate predicate _DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev_DataFlowDispatch::getNextClassInMro/1#__#shared/3@i6#L3#f893bw2h (iteration 6)
(377s) Tuple counts for _DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev_DataFlowDispatch::getNextClassInMro/1#__#shared/3@i6#L3#f893bw2h after 16ms:
33363  ~0%     {2} r1 = SCAN `DataFlowDispatch::getNextClassInMro/1#e1ee596a#prev_delta` OUTPUT In.1, In.0 'arg1'
159696 ~4%     {3}    | JOIN WITH `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev` ON FIRST 1 OUTPUT Rhs.1 'arg0', Lhs.1 'arg1', Rhs.2 'arg2'
               return r1
(377s) Starting to evaluate predicate _Class::Class.getAMethod/0#dispred#66416e47_Function::Function.getName/0#dispred#033700ef_10#join_rh__#antijoin_rhs/3@i6#L4#f893bw2h (iteration 6)
(382s) Tuple counts for _Class::Class.getAMethod/0#dispred#66416e47_Function::Function.getName/0#dispred#033700ef_10#join_rh__#antijoin_rhs/3@i6#L4#f893bw2h after 4.4s:
1770825904 ~4%     {4} r1 = JOIN `_DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev_DataFlowDispatch::getNextClassInMro/1#__#shared` WITH `Function::Function.getName/0#dispred#033700ef_10#join_rhs` ON FIRST 1 OUTPUT Lhs.1 'arg0', Rhs.1, Lhs.0 'arg1', Lhs.2 'arg2'
34558      ~3%     {3}    | JOIN WITH `Class::Class.getAMethod/0#dispred#66416e47` ON FIRST 2 OUTPUT Lhs.0 'arg0', Lhs.2 'arg1', Lhs.3 'arg2'
                   return r1
...
(382s) Starting to evaluate predicate DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3/3@i6#f893b1xh (iteration 6)
(382s)                     - DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3_delta has 125138 rows (order for disjuncts: delta=<standard>).
(382s) Tuple counts for DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3/3@i6#f893b1xh after 12ms:
33363  ~0%     {2} r1 = SCAN `DataFlowDispatch::getNextClassInMro/1#e1ee596a#prev_delta` OUTPUT In.1, In.0 'cls'
159696 ~0%     {3}    | JOIN WITH `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev` ON FIRST 1 OUTPUT Lhs.1 'cls', Rhs.1 'name', Rhs.2 'result'
125138 ~1%     {3}    | AND NOT `_Class::Class.getAMethod/0#dispred#66416e47_Function::Function.getName/0#dispred#033700ef_10#join_rh__#antijoin_rhs`(FIRST 3)

0      ~0%     {3} r2 = JOIN `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev_delta` WITH `DataFlowDispatch::getNextClassInMro/1#e1ee596a#reorder_1_0#prev` ON FIRST 1 OUTPUT Lhs.1 'name', Lhs.2 'result', Rhs.1 'cls'
               {3}    | AND NOT `_Class::Class.getAMethod/0#dispred#66416e47_DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#__#antijoin_rhs`(FIRST 3)
0      ~0%     {3}    | SCAN OUTPUT In.2 'cls', In.0 'name', In.1 'result'

125138 ~1%     {3} r3 = r1 UNION r2
125138 ~1%     {3}    | AND NOT `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev`(FIRST 3)
               return r3
```

And now
```
(18s) Tuple counts for DataFlowDispatch::class_has_method/2#0d2ae9c0/2@ff66c1lr after 18ms:
202279 ~1%     {2} r1 = JOIN `Class::Class.getAMethod/0#dispred#66416e47_10#join_rhs` WITH `Function::Function.getName/0#dispred#033700ef` ON FIRST 1 OUTPUT Lhs.1 'cls', Rhs.1 'name'
               return r1
...
(490s) Tuple counts for DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3/3@i6#48b6c1xi after 54ms:
0      ~0%     {3} r1 = JOIN `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev_delta` WITH `DataFlowDispatch::getNextClassInMro/1#e1ee596a#reorder_1_0#prev` ON FIRST 1 OUTPUT Rhs.1 'cls', Lhs.1 'name', Lhs.2 'result'
0      ~0%     {3}    | AND NOT `DataFlowDispatch::class_has_method/2#0d2ae9c0`(FIRST 2)

33363  ~0%     {2} r2 = SCAN `DataFlowDispatch::getNextClassInMro/1#e1ee596a#prev_delta` OUTPUT In.1, In.0 'cls'
159696 ~0%     {3}    | JOIN WITH `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev` ON FIRST 1 OUTPUT Lhs.1 'cls', Rhs.1 'name', Rhs.2 'result'
125138 ~1%     {3}    | AND NOT `DataFlowDispatch::class_has_method/2#0d2ae9c0`(FIRST 2)

125138 ~1%     {3} r3 = r1 UNION r2
125138 ~1%     {3}    | AND NOT `DataFlowDispatch::findFunctionAccordingToMro/2#a610c0a3#prev`(FIRST 3)
               return r3
```

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowDispatch.qll

@@ -829,10 +829,16 @@ Function findFunctionAccordingToMro(Class cls, string name) {
   result = cls.getAMethod() and
   result.getName() = name
   or
-  not cls.getAMethod().getName() = name and
+  not class_has_method(cls, name) and
   result = findFunctionAccordingToMro(getNextClassInMro(cls), name)
 }
 
+/**
+ * Join-order helper for `findFunctionAccordingToMro` and `findFunctionAccordingToMroKnownStartingClass`.
+ */
+pragma[nomagic]
+private predicate class_has_method(Class cls, string name) { cls.getAMethod().getName() = name }
+
 /**
  * Gets a class that, from an approximated MRO calculation, might be the next class
  * after `cls` in the MRO for `startingClass`.
@@ -860,7 +866,7 @@ private Function findFunctionAccordingToMroKnownStartingClass(
   result.getName() = name and
   cls = getADirectSuperclass*(startingClass)
   or
-  not cls.getAMethod().getName() = name and
+  not class_has_method(cls, name) and
   result =
     findFunctionAccordingToMroKnownStartingClass(getNextClassInMroKnownStartingClass(cls,
         startingClass), startingClass, name)