ICE on using result of index operation on a constant to index into a constant

``` rust
const ARR: [usize; 5] = [5, 4, 3, 2, 1];

fn main() {
    println!("{}", ARR[ARR[3]]);
}
```

causes

```
<anon>:5:24: 5:30 error: internal compiler error: index is not an integer-constant expression
<anon>:5     println!("{}", ARR[ARR[3]]);
                                ^~~~~~
```
