Warn about always true/false isinstance tests

[JukkaL]

These may result in code that mypy considers unreachable and silently doesn't type check. However, they may happen quite frequently in innocent things like assert isinstance(x, str) so finding a good heuristic for what to report can be tricky.

[rwbarton]

Initial reactions:

• Don't warn about an assertion that always passes
• Do warn about an assertion that always fails
• Do warn about a branch of an if that is unreachable

[ddfisher]

To add to Reid's comment: but don't warn about an assertion that always fails if it's an explicit assert False.

[refi64]

@ddfisher Or any false-y builtin constant, since I've seen assert 0 and assert None before.

[gvanrossum]

@ilinum This issue might be interesting given your recent quest for flagging unreachable code.

[ilevkivskyi]

I have found five other issues on the tracker that are some variations of this, so I think this is high priority.

[ilevkivskyi]

Also from my experience there is definitely no need in a warning about always True asserts. So 100% with @rwbarton above.