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

[Nikita Popov via llvm-commits]

nikic wrote:

@yingopq As I understand it, this was reproduced with the release/18.x branch (or rust's fork, which is essentially the same), which already contains that second commit as well.

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