[ConstraintElimination] Use logic from 3771310e for queries only.

The logic added in 3771310e was placed sub-optimally. Applying the
transform in ::getConstraint meant that it would also impact conditions
that are added to the system by the signed <-> unsigned transfer logic.

This meant we failed to add some signed facts to the signed system. To
make sure we still add as many useful facts to the signed/unsigned
systems, move the logic to the point where we query the system.