pop prefetch values from committed_state when they are available

pop prefetch values from committed_state when they are available

Fixed issue where when making use of the
:paramref:`_orm.relationship.post_update` feature at the same time as using
a mapper version_id_col could lead to a situation where the second UPDATE
statement emitted by the post-update feature would fail to make use of the
correct version identifier, assuming an UPDATE was already emitted in that
flush which had already bumped the version counter.

Fixes: #10800
Change-Id: I3fccdb26ebbd2d987bb4f0e894449b7413556054