defer: enable loop lowering

cl/_testgo/defercomplex/in.go

@@ -0,0 +1,51 @@
+package main
+
+func main() {
+	for _, label := range complexOrder() {
+		println(label)
+	}
+}
+
+func complexOrder() (res []string) {
+	record := func(label string) { res = append(res, label) }
+
+	defer record(label1("cleanup-final", 0))
+	defer record(label1("cleanup-before-loop", 0))
+
+	for i := 0; i < 2; i++ {
+		defer record(label1("exit-outer", i))
+		for j := 0; j < 2; j++ {
+			if j == 0 {
+				defer record(label2("branch-even", i, j))
+			} else {
+				defer record(label2("branch-odd", i, j))
+			}
+			for k := 0; k < 2; k++ {
+				nested := label3("nested", i, j, k)
+				defer record(nested)
+				if k == 1 {
+					defer record(label3("nested-tail", i, j, k))
+				}
+			}
+		}
+	}
+
+	defer record(label1("post-loop", 0))
+	return
+}
+
+func label1(prefix string, a int) string {
+	return prefix + "-" + digit(a)
+}
+
+func label2(prefix string, a, b int) string {
+	return prefix + "-" + digit(a) + "-" + digit(b)
+}
+
+func label3(prefix string, a, b, c int) string {
+	return prefix + "-" + digit(a) + "-" + digit(b) + "-" + digit(c)
+}
+
+func digit(n int) string {
+	return string(rune('0' + n))
+}