stop having identity casts be lexprs

that made no sense (see test), and was incompatible with borrowck.

Fixes #36936.

src/test/run-pass/issue-36936.rs

@@ -0,0 +1,35 @@
+// 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.
+
+// check that casts are not being treated as lexprs.
+
+fn main() {
+    let mut a = 0i32;
+    let b = &(a as i32);
+    a = 1;
+    assert_ne!(&a as *const i32, b as *const i32);
+    assert_eq!(*b, 0);
+
+    assert_eq!(issue_36936(), 1);
+}
+
+
+struct A(u32);
+
+impl Drop for A {
+    fn drop(&mut self) {
+        self.0 = 0;
+    }
+}
+
+fn issue_36936() -> u32 {
+    let a = &(A(1) as A);
+    a.0
+}