Add SPV_INTEL_subgroup_buffer_prefetch extension support (#3690)
`OpSubgroupBlockPrefetchINTEL` instruction prefetches a contiguous byte
block from CrossWorkgroup memory per subgroup invocation, with an
optional Memory Operands bitmask.
Spec:
https://github.com/KhronosGroup/SPIRV-Registry/blob/main/extensions/INTEL/SPV_INTEL_subgroup_buffer_prefetch.asciidoc
AI-assisted: Claude Sonnet 4.6 (commercial SaaS)
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/c8862dd465aba4a
