i128::saturating_add returns -1 instead of i128::MAX at -O2 with the gcc backend #1027

Repro

#[inline(never)]
fn f(x: &[u8; 16]) -> i128 { i128::from_ne_bytes(*x).saturating_add(1) }
fn main() { println!("{}", f(&i128::MAX.to_ne_bytes())); }

$ rustc +nightly -Zcodegen-backend=gcc -Copt-level=2 repro.rs && ./repro
-1

Expected 170141183460469231731687303715884105727 (i128::MAX), which is what -Copt-level=0/1, the LLVM backend at every level, and Miri print. -Copt-level=3 also prints -1.

Versions: rustc 1.102.0-nightly (5b7b37aef 2026-11-08), libgccjit 17.0.0 20261006, x86_64-unknown-linux-gnu. Wrong on every nightly that ships rustc-codegen-gcc (checked 2026-06-05, 2026-09-01, 2026-11-08).