Fix when yield from was made of an AnyType

[rockneurotiko]

As reported in #868 and the generic example that @nierob reported in #836

Now don't crash in:

import asyncio

@asyncio.coroutine
def example_coro():
    q = asyncio.Queue()
    msg = yield from q.get()

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(example_coro())

But that example is because don't infer asyncio.Queue type properly as
is appointed in #836, and q is AnyType (instead of asyncio.Queue)

and

import asyncio
import typing

@asyncio.coroutine
def foo(a):
   b = yield from a
   return b

[gladhorn]

I haven't looked into the issue much, but this commit fixes several crashes I was experiencing.

[JukkaL]

Thanks for the PR! It would be great if you could add a test case for this as well.

Also, this doesn't address the root problem, which is an unsafe cast in visit_yield_from_expr:

    def visit_yield_from_expr(self, e: YieldFromExpr) -> Type:
        result = self.accept(e.expr)
        result_instance = cast(Instance, result)   # result could be any type -- cast is unsafe!
        ...

This can still fail if the result type is something other than Any or an instance type.

[JukkaL]

Closing as #944 contained fixes to the issues.