8133456: HeapRegionManager::shrink_by() iterates suboptimally across regions Reviewed-by: brutisso, tbenson

8067336: Allow that PLAB allocations at the end of regions are flexible Summary: PLAB allocations may return a buffer that is between minimum size (current allocation) and the desired size. This allows removes a large amount of fragmentation at the end of regions. Reviewed-by: tbenson, mgerdin

Merge