[X86][NFC] Use mnemonic tables in validateInstruction 3/4

Group switch cases by opcode:
- V4FMADDPS
- V4FMADDSS
- V4FNMADDPS
- V4FNMADDSS
- VP4DPWSSDS
- VP4DPWSSD

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D127718