type_limits warning should fire for operators like (<=)

The following appropriately generates a type_limits warning:

```
pub fn main() {
    let x: u64 = 0;
    println!("is {} negative? {}", x, (x < 0));
}
```

The following does not:

```
pub fn main() {
    let x: u64 = 0;
    println!("is {} nonpositive? {}", x, (x <= 0));
}
```
