Modifies free spill slot selection logic to use is_our_spill_or_restore instead of the labels with spill slot use information added for this purpose. We do not need the extra information in the latter and can simply use the former routine.
Removes the extra spill slot use metadata added in form of labels from the instrlist.
Also adds documentation about possibility of DR slot conflicts if DR APIs are mixed with drreg ones.
Issue: #3823 (closed)