test: add test which crashed gccgo compiler

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5605046

test/fixedbugs/bug403.go

+// $G $D/$F.go
+
+// Copyright 2012 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.
+
+// Crashed gccgo.
+
+package p
+
+type S struct {
+	f interface{}
+}
+
+func F(p *S) bool {
+	v := p.f
+	switch a := v.(type) {
+	case nil:
+		_ = a
+		return true
+	}
+	return true
+}