RFR: JDK-8220664: Simplify ShenandoahUpdateHeapRefsClosure

[Roman Kennke]

We have split ShenandoahUpdateHeapRefsSuperClosure and
ShenandoahUpdateHeapRefsClosure in our code for no reason. We can fold
them into one class. (This is historical, we used to have a *Matrix
variant there.)

Bug:
https://bugs.openjdk.java.net/browse/JDK-8220664
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8220664/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman