When multiplanning is used, the deferred engine selection multiplanners move the winning QSN out of the multiplan stage. This means we have to move the QSN back into the MPS later on, during lowering.

We should consider matching what CBR does where we return a nullptr as the QSN, and have the lowering code move the QSN out of the MPS and then move it back into the MPS, so the ownership movement is closer together and more predictable.