is also the parent of the firstMatch-sj-nest

Check, and enforce, that the table to do firstMatch duplicate elimination
from can also be made the parent of firstmatch-semijoin in the pushed
joins.

Patch also contains some fixes to ::prepareResultSet.

Reviewed by: Mikael Ronstrom <mikael.ronstrom@oracle.com>

Change-Id: Ie437f73a0a1f6b92b5a060e97ab81d334ede6590
(cherry picked from commit e5385bbd3d42900b5d119c8b81f4204cf0f2db15)