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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Is there a reason mips is reinventing all this stuff in a machine pass? AtomicExpand should already handle all of this? 

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