[llvm] [AMDGPU] Fix conformance issue for ctlz. (PR #83170)

[Diana Picus via llvm-commits]

rovka wrote:

> Remove LSH transform and restore original lowering.
> 
> Fixes conformance issue in [77615](https://github.com/llvm/llvm-project/pull/77615).

This doesn't explain what the conformance issue is :) You should add a few words about that.

Also, does this really restore the original lowering? The original lowering was not custom, IIUC. It seems this patch only brings the custom lowering closer to the original one, but it's not entirely equivalent to a revert of #77615.

https://github.com/llvm/llvm-project/pull/83170