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

Group switch cases by opcode:
- VGATHERDPD
- VGATHERDPS
- VGATHERQPD
- VGATHERQPS
- VPGATHERDD
- VPGATHERDQ
- VPGATHERQD
- VPGATHERQQ

Distinguish masked vs non-masked forms by EVEX encoding.

Reviewed By: skan, craig.topper

Differential Revision: https://reviews.llvm.org/D127719