Driveby cleanup of the impl for negation, which had some kind of

surprising casts. This version more obviously corresponds to the builtin
semantics.

src/test/compile-fail/issue-13352.rs

@@ -23,4 +23,6 @@ fn main() {
         unsafe { libc::exit(0 as libc::c_int); }
     });
     2_usize + (loop {});
+    //~^ ERROR E0277
+    //~| ERROR E0277
 }