auto merge of #9174 : thestinger/rust/bot, r=catamorphism

An expression such as `bottom == not_bottom` or `not_bottom == bottom`
already compiled, but this fixes the case where both sides are `bottom`.

src/test/run-fail/binop-fail-3.rs

@@ -0,0 +1,13 @@
+// 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.
+
+// error-pattern:quux
+fn foo() -> ! { fail!("quux"); }
+fn main() { foo() == foo(); }