RFR: 8166811: Missing memory fences between memory allocation and refinement

[Yangfei (Felix)]

Hi,

Please review the backport of JDK-8166811 to 8u:

Original bug: https://bugs.openjdk.java.net/browse/JDK-8166811 
Original RFR thread: https://mail.openjdk.java.net/pipermail/hotspot-dev/2016-October/025027.html 
Original change: http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/075fbfdb498f 

This is necessary for RMO architectures like aarch64.
Original change does not apply cleanly to 8u-dev repo.  Two changes for 8u:
1. Removed comment added in file g1CollectedHeap.cpp;
2. Trivial adaptation is needed for newly added function is_old_or_humongous in file heapRegionType.hpp;

Webrev for 8u: http://cr.openjdk.java.net/~fyang/8259659/webrev.00/

Performed two rounds of jtreg testing: first round without extra option and second round with -XX:+UseG1GC option. 
Also performed two rounds of specjbb2015 with -XX:+UseG1GC. 
OK to backport? 

Thanks,
Felix