cmd/compile: add test for array decomposition

This test fails on 1.9.2, but is ok on tip.
CL 77331 has both the 1.9.2 fix and this test, and is on the 1.9 release branch.
This CL is just the test, and is on HEAD.  The buggy code doesn't exist on tip.

Update #22683

Change-Id: I04a24bd6c2d3068e18ca81da3347e2c1366f4447
Reviewed-on: https://go-review.googlesource.com/77332
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue22683.go

@@ -0,0 +1,30 @@
+// cmpout
+
+// Copyright 2017 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.
+
+package main
+
+import (
+	"fmt"
+)
+
+type foo struct {
+	bar [1]*int
+}
+
+func main() {
+	ch := make(chan foo, 2)
+	var a int
+	var b [1]*int
+	b[0] = &a
+	ch <- foo{bar: b}
+	close(ch)
+
+	for v := range ch {
+		for i := 0; i < 1; i++ {
+			fmt.Println(v.bar[0] != nil)
+		}
+	}
+}

test/fixedbugs/issue22683.out

@@ -0,0 +1 @@
+true