union with intersection doesn't narrow properly

upstream issue, but:
```py
class A: ...
class B: ...
class C: ...

other: (A & B) | C
data: B | C
data = other
if isinstance(data, A):
reveal_type(data) # Never
```
should be `(A & B) | (A & C)`

interestingly, this only occurs when it's narrowed and doesn't happen when you specify the type explicitly `data: (A & B) | C`

Start with the Python reproduction in the issue and trace the type-narrowing path that handles `isinstance(data, A)` for the union involving an intersection. Done means the revealed type is `(A & B) | (A & C)` rather than `Never`, while the explicitly annotated case continues to behave correctly.

Written by the indexing model from the issue text.