x86: use correct register names

x86: use correct register names

VEX.W may be legitimately set (and is then ignored by the CPU) for
non-64-bit code. Don't print 64-bit register names in such a case, by
utilizing that REX_W would never be set for non-64-bit code, and that
it is being set from VEX.W by generic decoding.

A test for this is going to be introduced in the next patch of this
series.