only consider column / relationship attrs for subclass IN

only consider column / relationship attrs for subclass IN

Fixed issue in  :func:`_orm.selectin_polymorhpic` loader option where
attributes defined with :func:`_orm.composite` on a superclass would cause
an internal exception on load.

Define the prop for :class:`.PropRegistry` as a
:class:`.StrategizedProperty`; we dont make path registries for
descriptor props like synonyms, composites, etc.

Fixes: #11291
Change-Id: I6f16844d2483dc86ab402b0b8b1f09561498aa1f