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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

LGTM, but really we should fix the defaults in the BasicTTIImpl

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