refine ruleset to determine when poly adaption should be used

refine ruleset to determine when poly adaption should be used

Fixed regression appearing in the 1.4 series where a joined-inheritance
query placed as a subquery within an enclosing query for that same entity
would fail to render the JOIN correctly for the inner query. The issue
manifested in two different ways prior and subsequent to version 1.4.18
(related issue #6595), in one case rendering JOIN twice, in the other
losing the JOIN entirely. To resolve, the conditions under which
"polymorphic loading" are applied have been scaled back to not be invoked
for simple joined inheritance queries.

Fixes: #8456
Change-Id: Ie4332fadb1dfc670cd31d098a6586a9f6976bcf7