[X86][NFC] Refine code in X86CompressEVEXTablesEmitter.cpp

1. Simplify getValueFromBitsInit about cast and return type
2. Remove out-of-date comments and allow memory ops in function
   object `IsMatch` so that we can reuse it for EVEX2Legacy compression.

This patch is to extract NFC in #77065 into a separate commit.