isinstance checks don't work with tuple types

[JukkaL]

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.