[X86] Add all of the MRM_C0-MRM_FF forms to the switch in...

[X86] Add all of the MRM_C0-MRM_FF forms to the switch in RecognizableInstr::emitInstructionSpecifier. NFC

Remove the special casing for MRM_F8 by using HANDLE_OPTIONAL.

This should be NFC as the forms that were missing aren't used by any instructions today. They exist in the enum so that we didn't have to put them in one at a time when instructions are added. But looks like we failed here.

llvm-svn: 327298