The main method for doing young collection (everything in G1CollectedHeap::do_collection_pause_at_safepoint()) should be reorganized a little - it's one of the larger methods in G1CollectedHeap code and over time got really messy.

There is a bit of everything here mixed together implicating dependencies where there are likely none. Group together related initialization (collection set, alloc regions, ...) a bit more.

(This is considered a first attempt on cleaning this problem up a little, not necessarily a full-scale refactoring)