return type on `isinstance` check with non-overlapping types is `bool` instead of `False`

### Describe the problem, ie expected/actual result (if it's not blatantly obvious)

this prevents you from being able to use `cast` to work around `redundant-expr` false positives (eg. #615)

### Gist to reproduce

```python
# mypy: enable-error-code=redundant-expr
from typing import cast

b = 1

reveal_type(isinstance(b, str)) # bool, should be False

if isinstance(b, str): # error: condition is always false
...

if cast(bool, isinstance(b, str)): # error: redundant expr
...
```

Begin with the supplied Python reproduction and compare the revealed type with the redundant-expr diagnostic. Locate the existing isinstance type-checking tests or implementation, then add regression coverage showing that non-overlapping types reveal False without breaking the existing diagnostic behavior.

Written by the indexing model from the issue text.