x86: re-work AVX-VNNI support

x86: re-work AVX-VNNI support

By putting the templates after their AVX512 counterparts, the AVX512
flavors will be picked by default. That way the need to always use {vex}
ceases to exist once respective CPU features (AVX512-VNNI or AVX512VL as
a whole) have been disabled. This way the need for the PseudoVexPrefix
attribute also disappears.