regression: error: @bitCast must have a known result type

Repro:

```c
#define BIT(n) (1u << (n))
#define BIT31 BIT(31)

unsigned int bit31() {
    return BIT31;
}
```

Translates to:

```zig
pub export fn bit31() c_uint {
    return @as(c_uint, 1) << @intCast(@bitCast(@as(c_int, @as(c_int, 31))));
}
```

Note the nested bitCast in an intCast.