"VEX.vvvv" field (VSR, VEX-specified-register) made explicit
in Optab encoding.
vexNDS, vexNDD, vexDDS and vexNOVSR do nothing,
this change does not produce any noticeable effect.
Rationale behind this change:
- keep more information inside optab entries
- make encodings match SDM more closely
- one less special rule to keep in mind
Pvex optabs are updated based on the Intel SDM descriptions.
Unused VEX combinations are removed;
it is problematic to choose VSR combinations for them
without actual Optabs that use them.
The origin of this idea can be found in:
https://go-review.googlesource.com/#/c/arch/+/66972/