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

[Alex Voicu via llvm-commits]

AlexVlx 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.

I have looked at the failure, it's rather odd; the log would suggest that the host's `opt` rather than the built `opt` is getting selected, but I'm having trouble repro-ing this locally. I can probably XFAIL / remove the test altogether as it's duplicating the SPIRV one anyway, just for the AMDGCNSPIRV triple, but that's somewhat unsatisfying.

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