DPE stats are computed when we have a dynamic partition selector that's
applied on another child of a join. The current code continues to use
DPE stats even for the common ancestor join and nodes above it, but
those nodes aren't affected by the partition selector.

Regular Memo groups pick the best expression among several to compute
stats, which makes row count estimates more reliable. We don't have
that luxury with DPE stats, therefore they are often less reliable.

By minimizing the places where we use DPE stats, we should overall get
more reliable row count estimates with DPE stats enabled.

The fix also ignores DPE stats with row counts greater than the group
stats. Partition selectors eliminate certain partitions, therefore
it is impossible for them to increase the row count.