[X86][CodeGen] Cleanup code for EVEX2VEX pass, NFCI

1. Remove unused variables, e.g X86Subtarget object in performCustomAdjustments
2. Define checkVEXInstPredicate directly instead of generating it b/c
   the function is small and it's unlikely we have more instructions to
   check the predicate in the future
3. Check the tables are sorted only once for each function
4. Remove some blanks and clang-format code