x86: drop Rdq, Rd, and MaskR

Rdq, Rd, and MaskR can be replaced by Edq, Ed / Rm, and MaskE
respectively, as OP_R() doesn't enforce ModRM.mod == 3, and hence where
MOD matters but hasn't been decoded yet it needs to be anyway. (The case
of converting to Rm is temporary until a subsequent change.)