test: add bug436, valid code that crashed gccgo

R=golang-dev, bsiegert, rsc
CC=golang-dev
https://golang.org/cl/6158046

test/fixedbugs/bug436.go

+// run
+
+// 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.
+
+// Gccgo used to crash compiling this.
+
+package main
+
+func foo() (int, int) {
+	return 1, 2
+}
+
+var c = b
+var a, b = foo()
+var d = b + 1
+
+func main() {
+	if a != 1 {
+		panic(a)
+	}
+	if b != 2 {
+		panic(b)
+	}
+	if c != 2 {
+		panic(c)
+	}
+	if d != 3 {
+		panic(d)
+	}
+}