[llvm] [llvm][SPIRV] Expose fast `popcnt` support for SPIR-V targets (PR #109845)

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy wrote:

LGTM overall, however, did you check failed Github action logs? I noticed that fails are related to this PR: `TEST 'LLVM :: Transforms/LoopIdiom/AMDGPU/popcnt.ll' FAILED` etc.

https://github.com/llvm/llvm-project/pull/109845