Integer constant treated as 16 bit

The following code prints 16959 instead of 999999 (999999 & 0xFFFF == 16959).

``` rust
fn main() {
    let mut count = 0;
    for 999_999.times() {
        count += 1;
    }
    io::println(fmt!("%u", count));
}
```

Tested as of 09bb07bed9166105ea961a42b5fff7739ae0d2e9 (I'm building tip now to reteset)
