test: fix bug423.go to actually fail with older releases.

The supposedly overflowing variable was registerized.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5687061

test/fixedbugs/bug423.go

@@ -14,7 +14,7 @@ func main() {
 }
 
 func F(arg int) {
-	var X int64
+	var X interface{}
 	_ = X // used once
 	X = 0
 	X = 0