`redundant-expr`/`unreachable` false positive when narrowing type with generic to subtype

```python
from typing import TypeVar, Generic, _T as T

class A(Generic[T]): ...
class B(A[int]): ...

a: A[object]
assert isinstance(a, B) # error: Intersection of "A[object] & B" cannot exist: would have inconsistent method resolution order [unreachable]
print("hi") # error: Statement is unreachable [unreachable]
```
[playground](https://mypy-play.net/?mypy=basedmypy-latest&python=3.12&gist=36f3c4f2cdb3a0e91b0c831ec4a236e8)

Reproduce the example in the linked mypy playground using Python 3.12 and the basedmypy-latest configuration. Trace how isinstance narrowing handles the generic A[object] and subtype B, then verify that the assertion is not reported as unreachable and the following print statement remains reachable.

Written by the indexing model from the issue text.