RFR: 8227637: Adjust Shenandoah C2 verifier to recognize IN_NATIVE barriers

[Roman Kennke]

 We get test failures in CTW tests because Shenandoah C2 verifier
doesn't recognize the new IN_NATIVE barriers.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8227637
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8227637/webrev.00/

Testing: hotspot_gc_shenandoah, ctw-tests

Can I please get a review?

(Aside: all this makes me think if this should be a special case of the
regular LRB instead, and become subject to the usual optos, etc. I need
to think this through. Let's fix the nightlies first.)

Roman