Dead code triggers mypyc assertion

```
def foo() -> None:
    while True:
        raise AssertionError
        continue
```
or
```
def foo() -> None:
    raise AssertionError
    x = 1
```
triggers a
```
asdf.py:3: AssertionError: Can't add to finished block
```

We should either produce a nice error, or alternately just allow it.