Reducing potential comm counts in dsiAdd

Thanks to @vasslitvinov for helping with this change.

Recent changes to dsiAdd resulted in higher comm counts for SSCA2_commDiags. By getting a const copy of the 'slotNum' argument, we can take advantage of remote value forwarding. We also restructure the call to _add based off of the 'findAgain' bool. These two changes should result in fewer comm counts, and in general a cleaner code structure.