fn vectorizable_iters(iters: &[Iter]) -> bool {
    iters.iter().all(|ref iter| {
        iter.start.is_none()
            && iter.end.is_none()
            && iter.stride.is_none()
            && match iter.data.ty {
                Vector(ref elem) if elem.is_scalar() => true,
                _ => false,
            }
    })
}
Returns `true` if this is a set of iterators we can vectorize, `false` otherwise.

We can vectorize an iterator if all of its iterators consume the entire collection.
weld/src/optimizer/transforms/vectorizer.rs