apply _path_with_polymorphic in prepend as well

apply _path_with_polymorphic in prepend as well

Fixed issue where using :meth:`_orm.Load.options` to apply a chained loader
option such as :func:`_orm.joinedload` or :func:`_orm.selectinload` with
:meth:`_orm.PropComparator.of_type` for a polymorphic relationship would
not generate the necessary clauses for the polymorphic subclasses. The
polymorphic loading strategy is now correctly propagated when using a call
such as ``joinedload(A.b).options(joinedload(B.c.of_type(poly)))`` to match
the behavior of direct chaining e.g.
``joinedload(A.b).joinedload(B.c.of_type(poly))``.

Fixes: #13202
Change-Id: I7b2ce2dd10a7f8583ff99495b0a65fa1a895ee29