Consider unsorted paths in generate_useful_gather_paths

 Consider unsorted paths in generate_useful_gather_paths
 
 generate_useful_gather_paths used to skip unsorted paths (without any
 pathkeys), but that is unnecessary - the later code actually can handle
 such paths just fine by adding a Sort node. This is clearly a thinko,
 preventing construction of useful plans.
 
 Backpatch to 13, where Incremental Sort was introduced.
 
 Author: James Coleman
 Reviewed-by: Tomas Vondra
 Backpatch-through: 13
 Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com