Fix Asserts in calc_non_nestloop_required_outer().

Fix Asserts in calc_non_nestloop_required_outer().

These were not testing the same thing as the comparable Assert
in calc_nestloop_required_outer(), because we neglected to map
the given Paths' relids to top-level relids.  When considering
a partition child join the latter is the correct thing to do.

This oversight is old, but since it's only an overly-weak Assert
check there doesn't seem to be much value in back-patching.

Richard Guo (with cosmetic changes and comment updates by me)

Discussion: https://postgr.es/m/CAMbWs49sqbe9GBZ8sy8dSfKRNURgicR85HX8vgzcgQsPF0XY1w@mail.gmail.com