test: add a test case that gccgo fails to compile

Change-Id: If36394e059cdae49834d26ad4ffdd3092a72a0b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/241997
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/bug509.go

@@ -0,0 +1,30 @@
+// compile
+
+// Copyright 2020 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 mishandles a couple of alias cases.
+
+package p
+
+type S struct{}
+
+func (*S) M() {}
+
+type I interface {
+	M()
+}
+
+type A = *S
+
+var V1 I
+var _ = V1.(*S)
+var _ = V1.(A)
+
+func F() {
+	var v I
+	v = (*S)(nil)
+	v = A(nil)
+	_ = v
+}