Add reqd_sub_group_size metadata for SubgroupSize (#3764)
Add function metadata without a vendor prefix for the `SubgroupSize`
Execution Mode, alongside the existing vendor-prefixed metadata.
`SubgroupSize` is a core SPIR-V feature, so representing it using
vendor-prefixed metadata was misleading.
Contributes to
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/issues/3742
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/3532d7454da6dba
