Question about PseudoInstExpansion

I am trying to do the following pseudo-to-real instruction conversion:

PseudoMFHI GPR32:$rd, ACC64:$ac → MFHI GPR32:$rd

Is there a way to tell the assembler, disassembler and instprinter to ignore the ACC64:$ac operand? I know I can use PseudoInstExpansion to accomplish this, but I just thought it would be convenient to have a flag for this, something like flag “DisableEncoding”. ACC64:$ac is just an artificial operand added to create a virtual register live range.

Also, the code in utils/TableGen/PseudoLoweringEmitter.cpp (near line 141) doesn’t seem to allow converting a pseudo to a code-gen-only instruction. What is the reason for this? Table-gen complains if I try to convert pseudo PseudoMFHI64 to code-gen-only instruction MFHI64 using PseudoInstExpansion:

PseudoMFHI64 GPR64:$rd, ACC128:$ac → MFHI64 GPR64:$rd