Fix an ICE when translating some static expressions.

Creating two identical static expressions involving casts of pointers to arrays
caused an assertion failure in librustc_trans.

src/test/run-pass/issue-21891.rs

@@ -0,0 +1,16 @@
+// Copyright 2015 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.
+
+static foo: [uint; 3] = [1, 2, 3];
+
+static slice_1: &'static [uint] = &foo;
+static slice_2: &'static [uint] = &foo;
+
+fn main() {}