DuplicateWeedout semi-join optimization requires that the tables in
the parent subquery provide rowids that can be compared across table
scans. Most engines support this, federated is the only exception.

DuplicateWeedout is the default catch-all semi-join strategy, which
must be always available. If it is not available for some edge case,
it's better to disable semi-join conversion altogether.

This is what was done in the fix for MDEV-30395. However that fix
has put the check before the view processing, so it didn't detect
federated tables inside mergeable VIEWs.

This patch moves the check to be done at a later phase, when mergeable
views are already merged.