Bool literals gt comparison causes a strange warning

The following program causes a strange warning when run on rust playground nightly:

``` rust
fn main() {
    true > false;
}
```

```
warning: can't do this op on bools, #[warn(const_err)] on by default
 --> <anon>:2:5
  |
2 |     true > false;
  |     ^^^^^^^^^^^^
```

However this version compiles cleanly:

``` rust
fn main() {
    let t = true;
    t > false;
}
```
