test: add bug439, valid code that caused a gccgo crash

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6198075

test/fixedbugs/bug439.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 crash compiling this.
+
+package p
+
+type E int
+
+func (e E) P() *E { return &e }
+
+const (
+	C1 E = 0
+	C2 = C1
+)
+
+func F() *E {
+	return C2.P()
+}