test: Add testcase which crashes gccgo.

R=rsc
CC=golang-dev
https://golang.org/cl/1995050

test/fixedbugs/bug304.go

+// $G $D/$F.go
+
+// Copyright 2010 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.
+
+// Caused a gccgo crash on compilation.
+// bug304.go: In function ‘p.f’:
+// bug304.go:15:2: internal compiler error: in copy_tree_r, at tree-inline.c:4114
+
+package p
+type S struct {
+	v interface{}
+}
+func g(e interface{}) { }
+func f(s S) {
+	g(s.v.(*int))
+}