test: add test that caused gccgo to crash on valid code

Update #8612

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/135170043

test/fixedbugs/issue8612.go

+//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.
+
+// Gccgo had a bug comparing a struct or array value with an interface
+// values, when the struct or array was not addressable.
+
+package p
+
+type A [10]int
+
+type S struct {
+	i int
+}
+
+func F1() S {
+	return S{0}
+}
+
+func F2() A {
+	return A{}
+}
+
+func Cmp(v interface{}) bool {
+	if F1() == v {
+		return true
+	}
+	if F2() == v {
+		return true
+	}
+	return false
+}