[llvm] [llvm] Add support for llvm IR atomicrmw fminimum/fmaximum instructions (PR #136759)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

LGTM. Only thing that appears to be missing is the main codegen bits, but best to leave that for the next patch 

https://github.com/llvm/llvm-project/pull/136759