Adds a test that demonstrates how to get the app value for a reg that was reserved in multiple phases using drreg. This is complex as in the current design a phase doesn't know about spills done in earlier phases. So, getting the app value in insertion phase for a reg that was reserved in app2app and insertion phases both requires both the phases to work together.
Adds to the documentation of drreg_get_app_value and drreg_statelessly_restore_app_value that they allow obtaining the reg's meta value from the last phase where it was reserved, OR the app value if there's no such previous phase.
Issue: #5121