G1: G1CollectionSet::finalize_young_part clears survivor list too early

In G1CollectionSet::finalize_young_part

```
// Clear the fields that point to the survivor list - they are all young now.
  survivors->convert_to_eden();
  .
  .
   double predicted_base_time_ms = _policy->predict_base_elapsed_time_ms(pending_cards);
```
clears the list of survivor regions before we predict the evacuation costs of these regions.

The predict_survivor_regions_evac_time(); called by predict_base_elapsed_time_ms attempts to iterate the survivor regions list after it has been cleared, thus it returns 0 survivor_regions_evac_time.