narrow types under same constant condition

```py
a: Final[str]

if a == "a":
def f(): ...
if a == "a":
f()
```
this shouldn't be an error

also `possibly-undefined`:
```py
def func(x: bool):
if x:
msg = 'hello world'
if x:
print(msg)
```

Reproduce both Python snippets in basedmypy and start by locating the checker logic for constant-condition narrowing and possibly-undefined diagnostics. Done means the repeated constant-condition function call and the second guarded use of msg no longer produce errors, with regression coverage for both cases.

Written by the indexing model from the issue text.