asyncio.coroutine not detected correctly on instance method invocation

[ncoghlan]

Similar to #996, I'm getting an incorrect not-an-iterable error on an @asyncio.coroutine call. In this case, the error seems to be a more convoluted one related to a particular combination of circumstances leading to the method definition not being detected as a coroutine.

The minimal reproducer I've come up with (along with a couple of examples of cases that work as expected) is the following:

import asyncio

@asyncio.coroutine
def _func_detected_as_cr():
    if (yield from asyncio.sleep(0.01)):
        return

class Example():

    @asyncio.coroutine
    def _method_detected_as_cr(self):
        if (yield from asyncio.sleep(0.01)):
            pass

    @asyncio.coroutine
    def passes_pylint_check_1(self):
        yield from _func_detected_as_cr()

    @asyncio.coroutine
    def passes_pylint_check_2(self):
        yield from self._method_detected_as_cr()

    @asyncio.coroutine
    def _method_not_detected_as_cr(self):
        if (yield from asyncio.sleep(0.01)):
            pass
            return

    @asyncio.coroutine
    def fails_pylint_check(self):
        yield from self._method_not_detected_as_cr()

That gives the result:

$ pylint -E spurious_pylint_E1133.py
************* Module spurious_pylint_E1133
spurious_pylint_E1133.py:31:19: E1133: Non-iterable value self._method_not_detected_as_cr() is used in an iterating context (not-an-iterable)```

As shown in the reproducer, the same code in a top level function gets correctly detected as a coroutine, while similar code is correctly detected as long as it doesn't have an early return in the if statement body.