x86: consistently print prefixes explicitly which are invalid with VEX etc

x86: consistently print prefixes explicitly which are invalid with VEX etc

All of data size, rep, lock, and rex prefixes are invalid with VEX- and
alike encoded insns. Make sure they get printed explicitly in all cases,
to signal the anomaly. With this, do away with "rex_ignored" - if there
is a rex prefix, we want to print it anyway for VEX etc (and there's
nothing "ignored" about it in the first place - such an instruction will
raise #UD).