restore transfer of mapper.local_table to DML for some cases

restore transfer of mapper.local_table to DML for some cases

Fixed regression appearing in 2.0.21 caused by :ticket:`10279` where using
a :func:`_sql.delete` or :func:`_sql.update` against an ORM class that is
the base of an inheritance hierarchy, while also specifying that subclasses
should be loaded polymorphically, would leak the polymorphic joins into the
UPDATE or DELETE statement as well creating incorrect SQL.

This re-introduces logic to set the `.table` of an ORM update or delete
back to `mapper.local_table` that was removed in d18ccdc997185b74 by
:ticket:`10279`; the logic is qualified to only take place for a
statement that's directly against a mapper and not one that's against
an aliased object.

Fixes: #11625
Change-Id: Ia228c99809370733f111925554167e39bcd6be1d