x86: fix 3-operand insn reverse-matching

The middle operand would have gone entirely unchecked, allowing e.g.

	vmovss		%xmm0, %esp, %xmm2

to assemble successfully, or e.g.

	vmovss		%xmm0, $4, %xmm2

causing an internal error. Alongside dealing with this also drop a
related comment, which hasn't been applicable anymore since the
introduction of 3-operand patterns with D set (and which perhaps never
had been logical to be there, as reverse-matched insns don't make it
there in the first place).