x86: Support Intel AVX VNNI

[Jan Beulich]

On 16.10.2020 20:07, H.J. Lu wrote:
> When AVX VNNI was added, we could either use different mnemonics
> from AVX512 VNNI or a {vex} prefix.  We went with {vex} and made it
> mandatory to avoid any confusion.

What confusion could there be when a person has given suitable
explicit .arch directives? And how is {vex3} in disassembler
output helping in any way, when comparing to all other AVX+
insns which have AVX512VL counterparts? (Apart from that I'd
further question why it needs to be {vex3} when {vex} would
suffice, but I'd like to see this dropped altogether anyway,
except perhaps in some non-default mode, where it then should
be output consistently.)

Jan