RFR: Shenandoah: Isolate changes around BarrierSetC2::enqueue_useful_gc_barrier()

[Roman Kennke]

This reverts the changes that we did to
BarrierSetC2::enqueue_useful_gc_barrier() and implements the equivalent
with the interfaces that exist in jdk11u.

In ShenandoahBarrierSetC2::enqueue_useful_gc_barrier() we establish
that we only have users that are SATB-pre-barrier-calls (CallLeafNode).
PhaseIGVN::add_users_to_worklist() would add the call *and* its
outgoing projections to the worklist. This is what I'm trying to do
here, so that we don't have to touch any shared code for that.

http://cr.openjdk.java.net/~rkennke/shjdk11-c2-enqueue-barrier/webrev.00/

Testing: hotspot_gc_shenandoah all good

Ok?

Roman