ICE when iterating over a u16 range whose bounds exceed 64k

[apoelstra]

An ICE is observable with this code:

fn part(_: u16) -> u32 {
    1
}

fn main() {
    for n in 100_000.. {
        let _ = part(n);
    }
}

thread 'rustc' panicked at 'index out of bounds: the len is 1 but the index is 1', src/librustc_lint/types.rs:79:12
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Changing part to take a u32 rather than u16 makes the ICE go away.

Playground link

[thepowersgang]

Investigation shows that 65536 will trigger the ICE, while 65535 will not

[estebank]

Introduced in 13962af#diff-d4213b0b0cef0ac9073ce6c7a3024192R79

cc @varkor

We can probably get away with doing an early return if eps.len() != 2 and no other changes.