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

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

The AMDGPUCodegenPrepare pass is scheduled only for the higher optimization levels and it isn't included in the -O0 pipeline. Isn't this patch relevant for -O0 compilation?

Nitpick:
Try to avoid the auto keyword.
Terminate all the comments with a period(.).

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