[SPIR-V] Map known opcodes for inline SPIR-V instructions to their actual type

[cassiebeckley]

#125316 adds an UnknownOp pseudoinstruction to the machine IR for the SPIR-V backend. This is intended to represent instructions generated by using inline SPIR-V, and enables vendors to author headers for their extensions adding support for instructions Clang and LLVM don't know about. But there are some use cases where HLSL users may want to create inline SPIR-V types representing types the SPIR-V backend is aware of, and in that case we may want to be able to look up the MIR type for a given type represented by an UnknownOp.