Fix segment reduction overflow.
Fix segment reduction overflow.

The `nsegments` argument type must be the same as the `segment_ids`
(since the latter are just indices into `nsegments`).  For large
inputs that would overflow `Index` (`long`) but still fit into `int64_t`,
this was previously causing a uncaught failure, since it would pass
the validation checks that used `int64_t`, but then would result in
a negative value when it's actually used.

Fixes #64023.
