[clang] [llvm] [AMDGPU] Match bitsin(typeof(x)) - popcnt(x) to s_bcnt0_i32 (PR #164847)

[Matt Arsenault via cfe-commits]

arsenm wrote:

Also should account for whatever DAGCombiner does, I'd expect it to have the same canonicalization guarded with some legality checks 

https://github.com/llvm/llvm-project/pull/164847