Support asymetric type-guards with any value in the false branch

[KotlinIsland]

def guard(x: object) -> x is int if True else x is str: ...
def guard(x: object) -> True if True else x is str: ...

likely a subset of #235

[DetachHead]

hmm, thinking about this more maybe it doesnt make sense to support it. i cant think of a situation where it'd be correct

def foo(value: object) -> value is str if True else value is int:
    """what code could possibly go here to make this typeguard correct in both cases?"""

[KotlinIsland]

hmm, thinking about this more maybe it doesnt make sense to support it. i cant think of a situation where it'd be correct

def foo(value: object) -> value is str if True else value is int:
    """what code could possibly go here to make this typeguard correct in both cases?"""

def f(x: object):
    if isinstance(x, str):
        return True
    if isinstance(x, int):
        return False
    raise Exception