Improve how JNIHandleBlock::oops_do distinguishes oops from non-oops

In JNIHandleBlock::oops_do(), the blocks may contain free list next pointers or oops. The distinction is detected by asking the CollectedHeap if the pointer value is in the heap reserve or not.

This forces GCs to have one single contiguous heap reservation, without holes in it, which seems like an unnecessary restriction.