[llvm] [Mips] Fix atomic min/max generate mips4 instructions when compiling for mips2 (PR #149983)

[via llvm-commits]

yingopq wrote:

> LGTM but we should probably do something about the atomic expand LLSC issue generically. AArch64 has that totally busted assumption about which RA is in use

Thanks, I will continue to investigate later.

https://github.com/llvm/llvm-project/pull/149983