RFR: Upstream merge 2018-06-01

[Roman Kennke]

Let's pull upstream changes up to:

changeset:   50225:a09af8ef8e5c
user:        roland
date:        Thu Apr 26 12:48:35 2018 -0700
summary:     8201367: assert(current != first_mem) failed: corrupted
memory graph in superword code

After that comes the C1 barriers modularization, and I'm going to take
this as single piece.

There've been a few trivial conflicts, but nothing in Shenandoah.

List of changes:
http://cr.openjdk.java.net/~rkennke/upstream-merge-2018-06-01/outgoing.txt


Testing: hotspot_gc_shenandoah looks good.

Ok?

Roman