[llvm] [AMDGPU]: Rewrite mbcnt_lo/mbcnt_hi to work item ID where applicable (PR #160496)

[Juan Manuel Martinez Caamaño via llvm-commits]

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

Just a minor comment, the rest looks good to me.

https://github.com/llvm/llvm-project/pull/160496