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

[Teja Alaghari via llvm-commits]

TejaX-Alaghari wrote:

@krzysz00 & @jayfoad, I've re-implemented the original mechanism in `AMDGPUCodeGenPrepare` now.

Can you please take a look again now and let me know your suggestions, thanks!

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