Support `or` in `isinstance` expressions.

[jhance]

I saw in some place in mypy something like

class A:
    pass

class B(A):
    pass

class C(A):
    pass

x = C()  # type: A
if isinstance(x, B) or isinstance(x, C):
    y = cast(Union[B, C], x)
    ...

We should be able to infer Union[B, C] as the type of x within this expression.

[JukkaL]

Yes, that would be nice. #947 is related (tuple as the second argument to isinstance).

[jhance]

It seems like this can get complex pretty quickly (imagine using tuples inside an and, now you effectively have an intersection of unions). Theres so many cases to cover that I'm thinking about just going all-out here and writing an all-encompassing solution.

I've been toying with converting the expressions to DNF and seeing if we can just do an all-encompassing solution that way but I'm not confident in its correctness since it effectively makes complicated cases just as difficult as simple cases.

[JukkaL]

At this point it's okay to add some limitations, such as for and all the operands must target different variables. So this wouldn't work correctly:

if isinstance(x, A) and isinstance(x, B): ...

(we'd only recognize the first isinstance, since mypy doesn't support intersection types)

We can revisit this later, but the complex cases are going to be pretty marginal so it's not worth it to complicate everything because of them.

[JukkaL]

In general, if in doubt whether a particular idiom or construct is worth supporting, try to find actual examples of that in some existing projects. If you can't find any (or just find a few isolated examples), it's probably not that important, and there are lot of things that mypy can't deal with that we know to be important (such as every issue tagged with 'priority').