Refactor G1 YC preparation code

During the review of JDK-8139424 an idea to move more of the preparation code for the G1 YC into G1CollectedHeap::pre_evacuate_collection_set() came up.

These are the methods that were discussed in the review:
g1_rem_set()->cleanupHRRS();
register_humongous_regions_with_cset();

But there is probably more preparation code that could be moved together.