RFR (xs): JDK-8214057: GC/C2 abstraction for Node::has_special_unique_user()

[Roman Kennke]

C2 needs to take special actions when a node has a special unique user.
Shenandoah has GC barrier nodes that need to do this. Let's put an
abstraction there.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8214057
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8214057/webrev.00/

Testing: tier1

Ok?

Roman

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