Overflow checking failed on #[repr(i64)] enums

Code:

``` rust
#[repr(i64)]
enum X {
    A = 0x_7FFF_FFFF_FFFF_FFFF,
    B,
}

fn main() {
    println!("{}", X::A as i64);
    println!("{}", X::B as i64);
}
```

Output:

```
9223372036854775807
-9223372036854775808
```

This is the opposite of https://github.com/rust-lang/rust/issues/23221
