test: add cases to return.go that gccgo got wrong

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12620043

test/return.go

@@ -1406,6 +1406,16 @@ L:
 	}
 } // ERROR "missing return"
 
+func _() int {
+	switch x.(type) {
+	default:
+		return 4
+	case int, float64:
+		print(2)
+		panic(3)
+	}
+}
+
 // again, with func literals
 
 var _ = func() int {
@@ -2798,4 +2808,14 @@ L:
 	}
 } // ERROR "missing return"
 
+var _ = func() int {
+	switch x.(type) {
+	default:
+		return 4
+	case int, float64:
+		print(2)
+		panic(3)
+	}
+}
+
 /**/