[llvm] [AMDGPU] Replace AMDGPUISD::FFBH_I32 with ISD::CTLS (PR #178420)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

> Can LowerCTLZ_CTTZ be replaced with generic legalization now?

I believe - partially. Per my understanding at i64 case handling for AMDGPU should diverge from a common handling.

https://github.com/llvm/llvm-project/pull/178420