test: add bug438, a valid test case that gccgo used to fail to compile

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6196047

test/fixedbugs/bug438.go

+// compile
+
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Gccgo used to incorrectly give an error when compiling this.
+
+package p
+
+func F() (i int) {
+	for first := true; first; first = false {
+		i++
+	}
+	return
+}