Change all survivor regions to eden at the end of gc

Currently all survivor regions are changed to eden at the beginning of gc.
As the retired survivor regions are not used any more but for memory tracking, the region relabelling can be moved to the end of gc.
This change simplifies survivor / eden region count and used bytes calculation.

Investigate its feasibility and change it.

This was discussed during JDK-8220089 offline discussion with Thomas S.