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

[via llvm-commits]

yingopq 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:
> 
> ```rust
> fn main() {
>     let x = std::sync::atomic::AtomicI8::new(23);
>     assert_eq!(x.fetch_max(42, std::sync::atomic::Ordering::SeqCst), 23);
> }
> ```
> 
> Please see [rust-lang/rust#123772](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.

hi, please use code https://github.com/llvm/llvm-project/commit/0e501dbd932ef1c6f4e747c83bf33beef0a09ecf to test again, because we submit another patch after https://github.com/llvm/llvm-project/commit/fbb27d16fa12aa595cbd20a1fb5f1c5b80748fa4. This issue include two patch.
Thanks, please let me know if you have any other questions.

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