RFR(XS): 8213898: CDS dumping of springboot asserts in G1ArchiveAllocator::alloc_new_region

[Jiangli Zhou]

Please review the following small fix for the 
assert(is_available(index)) failure when dumping the classes for 
springboot (thanks Eric Caspole for reporting the issue!).

   webrev: http://cr.openjdk.java.net/~jiangli/8213898/webrev.00/

   bug: https://bugs.openjdk.java.net/browse/JDK-8213898

HeapRegionManager::find_highest_free is used by 
G1ArchiveAllocator::alloc_new_region at CDS dump time for allocating a 
highest free GC region when archiving selected java heap objects. We 
need to check if the region obtained from the 
HeapRegionManager::_regions is available, as it might be uncommitted 
during heap shrinking. The fix has been reviewed by Thomas Schatzl in 
the bug report. Thanks Thomas!.

Verified the fix with springboot archiving. Tested with tier1-tier3.

Thanks,

Jiangli