[llvm] [AMDGPU] Add encoding/decoding support for non-result-returning ATOMIC_CSUB instructions (PR #68684)

[Jay Foad via llvm-commits]

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

LGTM modulo one inline nit. Please wait a day in case there are other comments.

https://github.com/llvm/llvm-project/pull/68684