RFR: Upstream merge jdk/jdk -> shenandoah/jdk 2018-09-11

[Roman Kennke]

Merging from upstream jdk/jdk up to tag jdk-12+11.

The only really interesting news is:
8210187: Explicit barriers for C2

Which resulted in some merging activity in C2 to fold exiting
resolve_for_*() into resolve() calls, but that was trivial.


List of changes:
http://cr.openjdk.java.net/~rkennke/jdk12-upstream-merge-2018-09-13/outgoing.txt

Testing: tier3_gc_shenandoah ok

Roman