`assert_eq!(-9223372036854775808isize as u64, 0x8000000000000000);`

fails on 32 bit and succeeds on 64 bit. These commits don't change that behavior.

The following worked before my changes, because the discriminant was
always processed as `u64`.
Now it fails, because the discriminant of `Bu64` is now `0` instead of `-9223372036854775808`. This is more in line with the above assertion's code, since
`-9223372036854775808isize as u64` on 32 bit yielded `0`.

```rust
enum Eu64 {
    Au64 = 0,
    Bu64 = 0x8000_0000_0000_0000
}
```