RFR: ShenandoahWriteBarrierRB flag to conditionally disable RB on WB fastpath

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/barrier-disable-wb-fastpath-rb/webrev.01/

I keep reimplementing this patch during performance investigations. It is sometimes useful to
dissect the WB cost by measuring if the evac-in-progress check itself, or the RB on the fastpath is
the reason for performance penalty. New flag allows to do that.

Testing: hotspot_gc_shenandoah, eyeballing assembly

Thanks,
-Aleksey