As it stands, the OperandMatchResultTy is only included in the generated header if there is custom operand parsing. However, almost all backends make use of MatchOperand_Success and friends from OperandMatchResultTy for e.g. parseRegister. This is a pain when starting an AsmParser for a new backend that doesn't yet have custom operand parsing. Move the enum to MCTargetAsmParser.h.
This patch is a prerequisite for D23563