txscript: Make isSmallInt accept raw opcode.

This converts the isSmallInt function to accept an opcode as a byte
instead of the internal opcode data struct in order to make it more
flexible for raw script analysis.

The comment is modified to explicitly call out the script version
semantics.

Finally, it updates all callers accordingly.

txscript/script.go

@@ -1,4 +1,5 @@
 // Copyright (c) 2013-2017 The btcsuite developers
+// Copyright (c) 2015-2019 The Decred developers
 // Use of this source code is governed by an ISC
 // license that can be found in the LICENSE file.
 
@@ -45,11 +46,12 @@ const (
 
 // isSmallInt returns whether or not the opcode is considered a small integer,
 // which is an OP_0, or OP_1 through OP_16.
-func isSmallInt(op *opcode) bool {
-	if op.value == OP_0 || (op.value >= OP_1 && op.value <= OP_16) {
-		return true
-	}
-	return false
+//
+// NOTE: This function is only valid for version 0 opcodes.  Since the function
+// does not accept a script version, the results are undefined for other script
+// versions.
+func isSmallInt(op byte) bool {
+	return op == OP_0 || (op >= OP_1 && op <= OP_16)
 }
 
 // isScriptHash returns true if the script passed is a pay-to-script-hash
@@ -136,7 +138,7 @@ func IsWitnessProgram(script []byte) bool {
 // bytes.
 func isWitnessProgram(pops []parsedOpcode) bool {
 	return len(pops) == 2 &&
-		isSmallInt(pops[0].opcode) &&
+		isSmallInt(pops[0].opcode.value) &&
 		canonicalPush(pops[1]) &&
 		(len(pops[1].data) >= 2 && len(pops[1].data) <= 40)
 }

txscript/standard.go

@@ -115,10 +115,10 @@ func isMultiSig(pops []parsedOpcode) bool {
 	if l < 4 {
 		return false
 	}
-	if !isSmallInt(pops[0].opcode) {
+	if !isSmallInt(pops[0].opcode.value) {
 		return false
 	}
-	if !isSmallInt(pops[l-2].opcode) {
+	if !isSmallInt(pops[l-2].opcode.value) {
 		return false
 	}
 	if pops[l-1].opcode.value != OP_CHECKMULTISIG {
@@ -153,7 +153,7 @@ func isNullData(pops []parsedOpcode) bool {
 
 	return l == 2 &&
 		pops[0].opcode.value == OP_RETURN &&
-		(isSmallInt(pops[1].opcode) || pops[1].opcode.value <=
+		(isSmallInt(pops[1].opcode.value) || pops[1].opcode.value <=
 			OP_PUSHDATA4) &&
 		len(pops[1].data) <= MaxDataCarrierSize
 }
@@ -705,7 +705,7 @@ func ExtractAtomicSwapDataPushes(version uint16, pkScript []byte) (*AtomicSwapDa
 			return nil, nil
 		}
 		pushes.SecretSize = int64(locktime)
-	} else if op := pops[2].opcode; isSmallInt(op) {
+	} else if op := pops[2].opcode; isSmallInt(op.value) {
 		pushes.SecretSize = int64(asSmallInt(op))
 	} else {
 		return nil, nil
@@ -716,7 +716,7 @@ func ExtractAtomicSwapDataPushes(version uint16, pkScript []byte) (*AtomicSwapDa
 			return nil, nil
 		}
 		pushes.LockTime = int64(locktime)
-	} else if op := pops[11].opcode; isSmallInt(op) {
+	} else if op := pops[11].opcode; isSmallInt(op.value) {
 		pushes.LockTime = int64(asSmallInt(op))
 	} else {
 		return nil, nil