opcode: optimize IsValid

Map access costs much more than array access.

name       old time/op  new time/op  delta
IsValid-8  17.6ns ± 2%   1.1ns ± 2%  -93.68%  (p=0.008 n=5+5)

pkg/vm/opcode/isvalid_test.go

@@ -0,0 +1,15 @@
+package opcode
+
+import (
+	"testing"
+)
+
+// IsValid() is called for every VM instruction.
+func BenchmarkIsValid(t *testing.B) {
+	// Just so that we don't always test the same opcode.
+	script := []Opcode{NOP, ADD, SYSCALL, APPEND, 0xff, 0xf0}
+	l := len(script)
+	for n := 0; n < t.N; n++ {
+		_ = IsValid(script[n%l])
+	}
+}

pkg/vm/opcode/opcode.go

@@ -222,8 +222,16 @@ const (
 	CONVERT Opcode = 0xDB
 )
 
+var validCodes [256]bool
+
+func init() {
+	// We rely on stringer here, it has a map anyway.
+	for op := range _Opcode_map {
+		validCodes[int(op)] = true
+	}
+}
+
 // IsValid returns true if the opcode passed is valid (defined in the VM).
 func IsValid(op Opcode) bool {
-	_, ok := _Opcode_map[op] // We rely on stringer here, it has a map anyway.
-	return ok
+	return validCodes[int(op)]
 }