test: add bug501.go, a gccgo compiler crash

Change-Id: I271707dee03fd97cd37bfb116eb00a5532034b57
Reviewed-on: https://go-review.googlesource.com/34020
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/bug501.go

+// run
+
+// Copyright 2016 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 got a compiler crash compiling the addition of more than five
+// strings with mixed constants and variables.
+
+package main
+
+func F(s string) (string, error) {
+	return s, nil
+}
+
+func G(a, b, c string) (string, error) {
+	return F("a" + a + "b" + b + "c" + c)
+}
+
+func main() {
+	if got, _ := G("x", "y", "z"); got != "axbycz" {
+		panic(got)
+	}
+}