Type inference with 'not isinstance' checks

Code like this currently fails but should be okay:

``` python
def f(x: Union[int, str]) -> None:
 if not isinstance(x, int):
 return
 x + 2 # x must be int here so this is okay, but mypy complains
```

Note that this works currently (without the `not` operator):

``` python
def f(x: Union[int, str]) -> None:
 if isinstance(x, str):
 return
 x + 2 # this is considered valid, as it should be
```
