Empty StmtSequences appear, for example, in the `else` branch of `if`
statements like the following:
foo
if cond
bar
else
end
baz
Before this change, the CFG for this code would look like this:
foo
│
│
▼
cond
│
true │
▼
bar
│
│
▼
if
│
│
▼
baz
i.e. there is linear flow through the condition, the `then` branch, and
out of the if. This doesn't account for the possibility that the
condition is false and `bar` is not executed. After this change, the CFG
looks like this:
foo
│
│
▼
cond
│ │
true │ │ false
▼ │
bar │
│ │
│ │
▼ ▼
if
│
│
▼
baz
i.e. we correctly account for the `false` condition.
This requires changing the CFG trees for classes and modules from
post-order to pre-order so that we can place the writes at the root node
of the tree, to prevent them overlapping with reads in the body of the
class/module.
We need to do this because classes and modules don't define their own
basic block, but re-use the surrounding one. This problem doesn't occur
for `self` variables in methods because each method has its own basic
block and we can place the write on the entry node of the bock.
