RFR: Bulk integration shenandoah/jdk8u -> aarch64-port/jdk8u-shenandoah 2019-04-29

[Roman Kennke]

Please review the following proposed changes:

This integrates changes from shenandoah/jdk8u into
aarch64-port/jdk8u-shenandoah repository, reaching back to December 2018.
It's bugfixes, improvements and new features that have been originally
done in Shenandoah dev repo, and then backported to Shenandoah JDK8u
repo and tested there for a while too.

Most of it is Shenandoah-only changes.

Short tour of shared-code changes:
- Change in classLoaderStats.cpp introduces a new required+missing 
object equals barrier.
- Change in subnode.cpp *reverts* back to orginal upstream code because 
of changed object equal barrier in C2.
- arguments.cpp change is in fact in Shenandoah-only path.

List of changes:
http://cr.openjdk.java.net/~rkennke/jdk8u-shenandoah-integration-2019-04-29/changes.txt

Complete Webrev:
http://cr.openjdk.java.net/~rkennke/jdk8u-shenandoah-integration-2019-04-29/webrev-all.00/

Thanks for reviewing!
Roman