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

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy 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.

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