[llvm] AMDGPU: Drop and upgrade llvm.amdgcn.atomic.csub/cond.sub to atomicrmw (PR #105553)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Could you be more specific about what tests are needed? If you could point me to the corresponding tests for one of the other atomicrmw operations (so that I can copy them and change the operation to usub_cond/usub_sat), that would be ideal.

Pretty much everything in test/Transforms/AtomicExpand.  expand-atomic-*-agent.ll and expand-atomic-*-system.ll are the most comprehensive tests 

https://github.com/llvm/llvm-project/pull/105553