x86: template-ize certain vector conversion insns

x86: template-ize certain vector conversion insns

Many of the vector conversion insns come with X/Y/Z suffixed forms, for
disambiguation purposes in AT&T syntax. All of these gorups follow
certain patterns. Introduce "xy" and "xyz" templates to reduce
redundancy.

To facilitate using a uniform name for both AVX and AVX512, further
introduce a means to purge a previously defined template: A standalone
<name> will be recognized to have this effect.

Note that in the course of the conversion VFPCLASSPH is properly split
to separate AT&T and Intel syntax forms, matching VFPCLASSP{S,D} and
yielding the intended "ambiguous operand size" diagnostic in Intel mode.