test: add a test that crashed gccgo

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13683046

test/fixedbugs/bug476.go

+// compile
+
+// Copyright 2013 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.
+
+// Logical operation on named boolean type returns the same type,
+// supporting an implicit convertion to an interface type.  This used
+// to crash gccgo.
+
+package p
+
+type B bool
+
+func (b B) M() {}
+
+type I interface {
+	M()
+}
+
+func F(a, b B) I {
+	return a && b
+}