fix regression on `@ptrCast`

this case regressed and now fixed:

```zig
const a: ?*i32 = undefined;
const b: ?*f32 = @ptrCast(?*f32, a);
```
e8bad1e12aAndrew Kelley committed on 8/23/2019, 9:39:56 PMparent101440c
1 file changedLine totals unavailable