```python
def f(b: bool) -> None:
    if b:
        y = 1
    else:
        y = None

f(False)
```

On encountering y = 1, mypyc uses the expression type to determine the
variable rtype. This usually works (as mypy infers the variable type
based on the first assignment and doesn't let it change afterwards),
except in this situation.

NameExpr(y)'s type is int, but the variable should really be int | None.
Fortunately, we can use the Var node's type information which is
correct... instead of blindly assuming the first assignment's type is
right.