all: VBMI2 instructions (#363)

Adds the "Vector Bit Manipulation Instructions 2" instruction set.

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

Contributed by @vsivsi. Extracted from #349 with simplifications.
Specifically, as prompted by the `dupl` linter we extract some common forms
lists into a helper `forms.go` file.

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

internal/opcodesextra/instructions.go

@@ -1,7 +1,11 @@
 // Package opcodesextra provides curated extensions to the instruction database.
 package opcodesextra
 
-import "github.com/mmcloughlin/avo/internal/inst"
+import (
+	"sort"
+
+	"github.com/mmcloughlin/avo/internal/inst"
+)
 
 // sets of extra instructions.
 var sets = [][]*inst.Instruction{
@@ -12,6 +16,7 @@ var sets = [][]*inst.Instruction{
 	vpclmulqdq,
 	vpopcntdq,
 	bitalg,
+	vbmi2,
 }
 
 // Instructions returns a list of extras to add to the instructions database.
@@ -34,5 +39,15 @@ func Instructions() []*inst.Instruction {
 			is = append(is, &c)
 		}
 	}
+
+	// Sort ISA lists. Similarly, this facilitates sharing helper functions for
+	// building forms lists without worrying about whether the ISA list is in
+	// the right order.
+	for _, i := range is {
+		for idx := range i.Forms {
+			sort.Strings(i.Forms[idx].ISA)
+		}
+	}
+
 	return is
 }