Thank you for the feedback Richard and Richard.

> Note the calls are guarded with
>
>       && ! known_eq (TYPE_VECTOR_SUBPARTS (vectype), 1U)

Yes, I believe nelt.is_constant() wouldn't be necessary. I didn't realize
the call was guarded by this condition.

> But I think the better check for location above is:
>
>    if (!multiple_p (nelt, 2))
>     return false;
>
> which then guards the assert in the later exact_div (nelt, 2).

I believe this check is better than using maybe_lt because it properly
guards exact_div(nelt, 2) and vec_perm_builder sel(nelt, 2, 3).
I'll modify the patch accordingly, build, test and submit the patch. Thank
you!!

Sincerely,