Change unreachable patterns ICEs to warnings

Allow code with unreachable `?` and `for` patterns to compile.
Add some tests.

src/test/compile-fail/unreachable-loop-patterns.rs

@@ -0,0 +1,20 @@
+// Copyright 2013 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.
+
+#![feature(never_type)]
+#![deny(unreachable_patterns)]
+
+fn main() {
+    let x: &[!] = &[];
+
+    for _ in x {}
+    //~^ ERROR unreachable pattern
+}
+