variable on right side of walrus operator is not narrowed

### Summary

```py
def foo(value: int | None):
    if foo := value:
        reveal_type(value)  # int | None, should be int
```

https://play.ty.dev/c4f152a1-89a6-4ea0-bed4-16c248dce13b

### Version

0.0.20