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

[Alex Voicu via llvm-commits]

AlexVlx wrote:

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

I'll pick your brains over this because I've not thought about this at all before you mentioned it:)

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