RFR: Clean up changes in phaseX.cpp

[Roman Kennke]

This cleans up our changes around has_special_unique_user(). Looks like
all uses of this method are followed by calls into BarrierSetC2, which
we can use to achieve what we need.

Testing: tier3_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/phasex-cleanups/webrev.00/

Ok?

Roman