Forward merging of trivial-reduction dims in producers (#1553)
* Forward merging of trivial-reduction dims in producers
* Enable trivial reduction forwarding only when trivial reduction domain
is a root domain.
For example, splitting a reduction domain by 1 and merging it with
another non-reduction domain would result in a trivial-reduction merge.
Probably possible to allow such non-root trivial reduction domains, but that
would mean, e.g., a leaf domain would be mappable yet its root domain
could be unmappable, which seems rather confusing. Considering such
transformations would be unlikely, not enabling forwarding would be fine
and would cause less surprise.
