isinstance checks don't work with tuple types

An invalid type is inferred for `x` after `isinstance` in this example:

```
from typing import Union, Tuple

def f(x: Union[str, Tuple[str, str]]) -> None:
    if isinstance(x, tuple):
        reveal_type(x)  # Revealed type is 'builtins.None'
```

This was originally reported by @davire at gitter.