[SPIR-V] OpPhi not handled as a phi node by the MIR verifier

[Keenuts]

When expensive checks are enabled, MIR is checked after each pass.
SPIR-V has an OpPhi instruction, similar to LLVM's phi instruction.
But because the MIR verifier doesn't know about it, it expects all operands to be defined by a dominating block (unlike the phi instruction).

Might want to add a new bit in the td files (akin to isBarrier bit) so the verifier can know "this instruction is allowed to have partially undefined operands)?

[Keenuts]

MIR verifier tests now pass with expensive checks enabled thanks to broxigarchen@affad1b