x86: template-ize vector packed byte/word integer insns

x86: template-ize vector packed byte/word integer insns

Many of the vector integer insns come in byte/word element pairs. Most
of these pairs follow certain encoding patterns. Introduce a "bw"
template to reduce redundancy.

Note that in the course of the conversion
- the AVX VPEXTRW template which is not being touched needs to remain
  ahead of the new "combined" ones, as (a) this should be tried first
  when matching insns against templates and (b) its Load attributes
  requires it to be first,
- this add a benign/meaningless IgnoreSize attribute to the memory form
  of PEXTRB; it didn't seem worth avoiding this.