Cherry-pick of upstreamed LLVM patch that fix MIPS int shift miscompilation

[martn3]

Summary

This PR is a cherry-pick of an LLVM upstream patch that fixes the MIPS miscompilation reported in rust-lang/rust#116177.

Details

The Rust git master branch miscompiles the following program for --target mips[el]-unknown-linux-gnu. The program comes from the above Rust issue that reported the problem.

main.rs

use std::hint::black_box;

const SHIFTVALUE : i64 = 10000000000i64;

fn main() {

    fn shiftleft(a : i64, b : i64) -> i64 {
        a << b
    }

    assert_eq!(shiftleft(black_box(SHIFTVALUE), black_box(2)), SHIFTVALUE << 2);
}

How to reproduce

To reproduce the miscompilation for MIPS with qemu on a Debian 12 x86_64 host with the Rust git master branch, do the following:

sudo apt install qemu-user gcc-mips-linux-gnu

./x build \
    --set llvm.download-ci-llvm=false \
    --set llvm.targets="Mips;X86" \
    --target mips-unknown-linux-gnu

./build/x86_64-unknown-linux-gnu/stage1/bin/rustc \
    --target mips-unknown-linux-gnu \
    -Clinker=mips-linux-gnu-gcc \
    -Ctarget-feature=+crt-static \
    main.rs

qemu-mips ./main

The assert should pass, but it fails with:

thread 'main' panicked at main.rs:11:5:
assertion `left == right` failed
  left: 35705032704
 right: 40000000000
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

How to fix

The miscompilation was fixed by the LLVM PR llvm#71149. This PR cherry-picks that commit. (The upstream LLVM issue that was fixed is llvm#71142. I believe it also fixed the LLVM issue llvm#64794 even though it is still open.)

I have confirmed that the Rust test program pass after applying the cherry-picked patch to the Rust LLVM fork and building with Rust git master.

[nikic]

This needs an upstream backport first, already requested on llvm#64794.