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

[Alex Voicu via llvm-commits]

AlexVlx wrote:

> The first concern coming to my mind is that we would need to have a test case with LIT CHECK's of the output SPIR-V to see/validate how ctpop and G_CTPOP are being lowered. We have test/CodeGen/SPIRV/llvm-intrinsics/ctpop.ll, but I wonder would it make sense to add a full-blown test case from unoptimized llvm ir to spir-v, what do you think?

I think this makes sense, can never have too many tests; we can probably mirror the `popcnt.ll` we currently have under LoopIdiom into a corresponding end-to-end test, I'll update this PR.

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