(🐞) false narrowing with reassignment in loop

```py
a: int | None
a = None
for i in range(10):
    reveal_type(a)  # "None" and also "int | None"
    if a:
        print(a)  # error: unreachable
    a = i
```

Also:
```py
a = None
for b in [1, 2]:
    if a is None:
        a = b
        continue
    print("hi")  # false unreachable error
```