Incorrect niche filling if enum contains both i32 min and i32 max variants

[dtolnay]

#![allow(dead_code)]

#[derive(Debug)]
#[repr(i32)]
enum E {
    Min = -2147483648i32,
    Max = 2147483647i32,
}

fn main() {
    println!("{:?}", Some(E::Min));
}

Rust 1.23.0 correctly prints Some(Min). Rust 1.24.0 incorrectly prints None.

Mentioning @eddyb because this sounds potentially relevant to #45225 or follow-on optimization.

(Discovered by @udoprog in #rust.)

[glandium]

Clearly from #45225 rather than a followup, as it's happening in nightly-2017-11-21 and not in nightly-2017-11-20.

[nox]

This is not specific to i32 but to the enum using both signed extremums of its representation type:

#[derive(Debug)]
#[repr(i8)]
enum E {
    Min = -128i8,
    Max = 127i8,
}