in group then group

[DenisYaroshevskiy]

• in group then group.

The common group of shuffles is: shuffle in groups, then shuffle groups.
Example is avx2 shuffle 128 byte lanes and then intermix them somehow.

There is a question: what to do first: do you shuffle big groups first or in big groups first.
Techincally: big groups first is strictly more powerful.

However, can lead to better 0ing.

It's not an obvious thing at all and I suspect we will come back to this one again.

Now I implemented "in groups then groups" to solve avx2 slide (partially.

• asimd

for asimd we used to have purely vextq_u16 based solution.
Now this will prefer shift_n when that's avaliable, becasue no constants (horray - the system works).

For neon it's not enabled, because there is a bug in level computation: emulation should always return 0 but sometimes it returns 1. Which messes with neon, double.