txscript: Optimize ExtractPkScriptAddrs pubkeyhash.

This continues the process of converting the ExtractPkScriptAddrs
function to use the optimized extraction functions recently introduced
as part of the typeOfScript conversion.

In particular, this converts the detection for pay-to-pubkey-hash
scripts.

txscript/standard.go

@@ -805,6 +805,19 @@ func PushedData(script []byte) ([][]byte, error) {
 	return data, nil
 }
 
+// pubKeyHashToAddrs is a convenience function to attempt to convert the
+// passed hash to a pay-to-pubkey-hash address housed within an address
+// slice.  It is used to consolidate common code.
+func pubKeyHashToAddrs(hash []byte, params *chaincfg.Params) []btcutil.Address {
+	// Skip the pubkey hash if it's invalid for some reason.
+	var addrs []btcutil.Address
+	addr, err := btcutil.NewAddressPubKeyHash(hash, params)
+	if err == nil {
+		addrs = append(addrs, addr)
+	}
+	return addrs
+}
+
 // scriptHashToAddrs is a convenience function to attempt to convert the passed
 // hash to a pay-to-script-hash address housed within an address slice.  It is
 // used to consolidate common code.
@@ -827,6 +840,11 @@ func ExtractPkScriptAddrs(pkScript []byte, chainParams *chaincfg.Params) (Script
 	// Avoid parsing the script for the cases that already have the able to
 	// work with raw scripts.
 
+	// Check for pay-to-pubkey-hash script.
+	if hash := extractPubKeyHash(pkScript); hash != nil {
+		return PubKeyHashTy, pubKeyHashToAddrs(hash, chainParams), 1, nil
+	}
+
 	// Check for pay-to-script-hash.
 	if hash := extractScriptHash(pkScript); hash != nil {
 		return ScriptHashTy, scriptHashToAddrs(hash, chainParams), 1, nil
@@ -849,18 +867,6 @@ func ExtractPkScriptAddrs(pkScript []byte, chainParams *chaincfg.Params) (Script
 	scriptClass := typeOfScript(scriptVersion, pkScript)
 
 	switch scriptClass {
-	case PubKeyHashTy:
-		// A pay-to-pubkey-hash script is of the form:
-		//  OP_DUP OP_HASH160 <hash> OP_EQUALVERIFY OP_CHECKSIG
-		// Therefore the pubkey hash is the 3rd item on the stack.
-		// Skip the pubkey hash if it's invalid for some reason.
-		requiredSigs = 1
-		addr, err := btcutil.NewAddressPubKeyHash(pops[2].data,
-			chainParams)
-		if err == nil {
-			addrs = append(addrs, addr)
-		}
-
 	case WitnessV0PubKeyHashTy:
 		// A pay-to-witness-pubkey-hash script is of thw form:
 		//  OP_0 <20-byte hash>