Fixed incorrect register move

tests/programs/op-assign.q

@@ -0,0 +1,9 @@
+main() {
+	f(10)
+}
+
+f(new) {
+	old := new
+	new -= 1
+	assert new != old
+}

tests/programs_test.go

@@ -26,6 +26,7 @@ var programs = []struct {
 	{"return", "", "", 0},
 	{"math", "", "", 0},
 	{"precedence", "", "", 0},
+	{"op-assign", "", "", 0},
 	{"binary", "", "", 0},
 	{"octal", "", "", 0},
 	{"hexadecimal", "", "", 0},