[HLSL][SPIRV] Handle 16/64-bit versions of `countbits`

[inbelic]

https://godbolt.org/z/cb9KGh5Eq

As shown in the reproduction case, we will generate an OpBitCount with 16/64-bit width types.

However, as denoted here, the SPIR-V validator will only accept these arguments to have a type of 32-bit width.

Note: This is in contrast to the SPIR-V OpBitCount specification that denotes the Result and Base type can be a scalar or vector of integer type.

As such, the expected behaviour should be such that it will emulate the operation for the non 32-bit types.

AC:

• Update lowering of bitcount intrinsic to emulate the non 32-bit operations similar to firstbitlow and firstbithight
• Add sufficient test cases, this may depend on: [HLSL][SPIRV] Clang should run spirv-val if available #142669.