Unreachable generator expressions fails to compile

This fails to compile:

```py
import sys
C = sys.platform == 'x' and (x for y in z)
```

There is a commented-out test in https://github.com/python/mypy/pull/10228/files#diff-456dccabc6ffa642d1668b9bcb96cf3aece3d132d4d7e64f66ee84b1783ad8f8R1070.

python/mypy#10228 fixed some issues with unreachable expressions, but not this one.