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

This adds the TTI predicate for conveying the availability of fast
`popcnt`, which subsequently allows passes like `LoopIdiomRecognize` to
collapse known popcount patterns. Since SPIR-V natively exposes
`OpBitcount`, it seems preferable to compress the resulting code, and
retain the information, even if a concrete target might have to expand
back into a loop structure.