Allow allocation into the tail end of humongous objects

A regular issue users have with G1 is that significant use of humongous objects causes additional memory pressure (compared to other collectors).

The last region of the humongous object often contains a large amount of free unusable space. Due to poor sizing of those by applications, they are very often almost completely free.

This enhancement should lift the restriction of not being able to allocate into the unused tail end of a large object during GC. Also, these tail regions should be allowed for evacuation (only evacuating the objects in the tail of course).