RFR (XXS): 8133456: HeapRegionManager::shrink_by() iterates suboptimally across regions

[Thomas Schatzl]

Hi all,

  can I have reviews for this one-line change that improves efficiency
during heap shrinking?

The problem was that the increment to skip over already scanned regions
has been too conservative. In some cases it would mean that regions in
the region table would be scanned twice.

CR:
https://bugs.openjdk.java.net/browse/JDK-8133456
Webrev:
http://cr.openjdk.java.net/~tschatzl/8133456/webrev/
Testing:
jprt, vm.gc 

Thanks,
  Thomas