test: add test case that gccgo miscompiled

Change-Id: I384eac632a4a87c12977e56a7d7bad7614305c51
Reviewed-on: https://go-review.googlesource.com/25143
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/bug498.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 incorrectly rejected an assignment to multiple instances of
+// the same variable.
+
+package main
+
+var a int
+
+func F() {
+	a, a, a = 1, 2, 3
+}
+
+func main() {
+	F()
+	if a != 3 {
+		panic(a)
+	}
+}