generic with constraints fails to narrow when using conditional expression

```py
from typing import TypeVar

class A:
    a: int
class B:
    b: int

T = TypeVar("T", A, B)

def foo(t: T):
    t.b if isinstance(t, B) else t.a # error: "B" has no attribute "a"
    
    if isinstance(t, B):
        t.b # no error
    else:
        t.a
```
https://mypy-play.net/?mypy=0.961&python=3.10&gist=1646ac27c8249ac4752a973d9b2b54d4