x86/APX: VROUND{P,S}{S,D} can generally be encoded

VRNDSCALE{P,S}{S,D} is the AVX512 generalization of these AVX insns. As
long as the immediate has the top 4 bits clear, they are equivalent to
the earlier VEX-encoded insns, and hence can be used to permit use of
eGPR-s in the memory operand. Since this is the normal way of using
these insns, also alter the resulting diagnostic to complain about the
immediate, not the eGPR use.