RFR: Load reference barriers

[Roland Westrelin]

> http://cr.openjdk.java.net/~rkennke/load-ref-barriers/webrev.00/

In ShenandoahBarrierSetC2::final_graph_reshaping() for case
Op_ShenandoahLoadReferenceBarrier: shouldn't we have:

assert(false, "should have been expanded already");

?

ShenandoahBarrierC2Support::call_wb_stub(): should that be renamed?
wb_mem seems useless?

Otherwise looks good to me.

Roland.