test: add missing copyright header to checkbce.go

Change-Id: Iafeb8e033c876f482caa17cca414fe13b0fadb12
Reviewed-on: https://go-review.googlesource.com/43613
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

test/checkbce.go

 // +build amd64
 // errorcheck -0 -d=ssa/check_bce/debug=3
 
+// Copyright 2016 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.
+
+// Test that the compiler does bounds check elimination as expected.
+// This avoids accidental regressions.
+
 package main
 
 func f0(a []int) {