[llvm] [mlir] Add usub_cond and usub_sat operations to atomicrmw (PR #105553)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Ok, I have created #105568 for the first commit. Once that has landed, this pull request will just be the AMDGPU change.

Can you change the title and rebase this one to include the target changes? I'm getting confused by the 2 PRs with the same   name 

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