test: add another test case that gccgo crashed on

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/124020044

test/fixedbugs/bug490.go

@@ -0,0 +1,16 @@
+// compile
+
+// Copyright 2014 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.
+
+// The gccgo compiler used to crash building a comparison between an
+// interface and an empty struct literal.
+
+package p
+ 
+type S struct{}
+
+func F(v interface{}) bool {
+	return v == S{}
+}