all: VNNI instructions (#359)

Adds "Vector Neural Network Instructions" instruction set.

These are added via the `opcodesextra` mechanism #345, since they're missing
from the opcodes database.

Contributed by @vsivsi. Extracted from #349 with some tweaks.

Co-authored-by: Vaughn Iverson <vsivsi@yahoo.com>

internal/opcodesextra/instructions.go

@@ -8,6 +8,7 @@ var sets = [][]*inst.Instruction{
 	movlqzx,
 	gfni,
 	vaes,
+	vnni,
 }
 
 // Instructions returns a list of extras to add to the instructions database.