RFR: Upstream merge 2018-06-21

[Roman Kennke]

Let's merge from upstream up to:

changeset:   51033:21b96ce2ed10
user:        stefank
date:        Tue Jun 19 08:44:31 2018 +0200
summary:     8205163: ZGC: Keeps finalizable marked PhantomReference
referents strongly alive

This brings lots of goodies. Notable for Shenandoah:
- 8203157: Object equals abstraction for BarrierSetAssembler
- 8204685: Abstraction for TLAB dummy object
- 8203641: Refactor String Deduplication into shared
- 8202845: Refactor reference processing for improved parallelism
- 8043575: Dynamically parallelize reference processing work
- 8204240: Extend MDO to allow more reasons to be recorded per bci


Next one up would be:
changeset:   50623:5209d8a6303e
user:        roland
date:        Tue Jun 19 09:08:39 2018 +0200
summary:     8203197: C2: consider all paths in loop body for loop
predication

... which makes messy conflicts and Roland asked me to resolve himself
in single step.

http://cr.openjdk.java.net/~rkennke/upstream-merge-2018-06-21/outgoing.txt

Testing: tier3_gc_shenandoah fastdebug