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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> And I think all the `auto`s in the PR are legitimate - they're generally appearing next to `dyn_cast<>` that already include the type

There are ones that are not 

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