accommodate subclass mapper in post-loader entity_isa check

accommodate subclass mapper in post-loader entity_isa check

Fixed issue where using chained loader options such as
:func:`_orm.selectinload` after :func:`_orm.joinedload` with
:meth:`_orm.PropComparator.of_type` for a polymorphic relationship would
not properly apply the chained loader option. The loader option is now
correctly applied when using a call such as
``joinedload(A.b.of_type(poly)).selectinload(poly.SubClass.c)`` to eagerly
load related objects.

Fixes: #13209
Change-Id: I2d14838f1b1a9a2b18dc52137910dab0bccf0dd5
(cherry picked from commit 2ac8c1a7c71400c1bc13f54f3c05cfd2e3ae1442)