Upstream merge jdk/jdk12 > sh/jdk12 to jdk-12+29

[Roman Kennke]

See $SUBJECT.

http://cr.openjdk.java.net/~rkennke/upstream-jdk12-merge-2019-01-25/changes.txt

- 8217467: Access barriers are missing in C2 intrinsic for Base64

Merged trivially

Testing: hotspot_gc_shenandoah (ongoing)

Ok?

Roman