all: VPOPCNTDQ instructions (#361)

Adds the VPOPCNTDQ instruction set, providing packed population count for
double and quadword integers.

These are added via the `opcodesextra` mechanism #345, since they're missing
from the opcodes database. In this case the 512-bit non-AVX512VL forms are
added here as well as the opcodes database, but they're deduplicated later.

Contributed by @vsivsi. Extracted from #234 with simplifications for AVX-512
form expansion.

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

internal/opcodesextra/instructions.go

@@ -10,6 +10,7 @@ var sets = [][]*inst.Instruction{
 	vaes,
 	vnni,
 	vpclmulqdq,
+	vpopcntdq,
 }
 
 // Instructions returns a list of extras to add to the instructions database.