[llvm] [SPIR-V][Codegen] Add isPhi bit to MCInstrDesc/MIR definitions (PR #110019)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

How does PHI end up transforming into an OpPhi? Can you directly consume generic PHI instructions, and only change the final encoding? 

https://github.com/llvm/llvm-project/pull/110019