x86: improve/shorten vector zeroing-idiom optimization conditional

- Drop the rounding type check: We're past template matching, and none
  of the involved insns support embedded rounding.
- Drop the extension opcode check: None of the involved opcodes have
  variants with it being other than None.
- Instead check opcode space, even if just to be on the safe side going
  forward.
- Reduce the number of comparisons by folding two groups.