Add regression tests for #13727.

Fixes https://github.com/rust-lang/rust/issues/13727.

src/test/compile-fail/issue-13727.rs

@@ -0,0 +1,22 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn test(val: u8) {
+  match val {
+    256 => print!("0b1110\n"),
+    512 => print!("0b1111\n"),
+    //~^ ERROR: unreachable pattern
+    _   => print!("fail\n"),
+  }
+}
+
+fn main() {
+  test(1);
+}