Inconsistent local variable narrowing for `finally` block

Consider the following:

```python
x = None
try:
x = 5
finally:
print(x)
reveal_type(x) # Literal[5] | None
```

If I change `print(x)` to `print(None)`, the revealed type changes.

This is due to a bad interaction between the handling of the "finally gate" in the code flow graph and an code flow optimization that skips blocks if the expression is not referenced. If `x` is not referenced in the `finally` block, we get a different answer.

Start by reproducing the issue's Python snippet and comparing the revealed types with and without the reference in the finally block. Trace the interaction between the finally gate in the code flow graph and the optimization that skips unreferenced expressions; done means both forms produce consistent narrowing.

Written by the indexing model from the issue text.