[llvm] [Mips] Fix missing sign extension in expansion of sub-word atomic max (PR #77072)

[Martin Nordholts via llvm-commits]

martn3 wrote:

Hello, I started to see MIPS failures in the Rust standard library tests after this change. For example, this program begins to fail after this change:

```rs
fn main() {
    let x = std::sync::atomic::AtomicI8::new(23);
    assert_eq!(x.fetch_max(42, std::sync::atomic::Ordering::SeqCst), 23);
}
```

Please see https://github.com/rust-lang/rust/issues/123772 for more details.

This seems quite problematic, so please let me know if I can help investigate this somehow.

https://github.com/llvm/llvm-project/pull/77072