[ty] Expand support for narrowing within walruses (#24968)
## Summary
Support narrowing for a few more already-supported sites, but in the
context of a walrus, as in:
```python
def f(t: tuple[int, int] | tuple[None, None]):
if (first := t[0]) is not None:
reveal_type(first) # int
reveal_type(t) # tuple[int, int]
else:
reveal_type(first) # None
reveal_type(t) # tuple[None, None]
```
