RFR: Implement ShenandoahBarrierSetC2::step_over_gc_barrier() to fix lock elimination bug with Shenandoah

[Aleksey Shipilev]

On 5/3/19 4:46 PM, Roman Kennke wrote:
> This fixes the lock elimination bug that we have seen and fixed already
> in sh/jdk8 and >11. We also did the fix in sh/jd11 but the bug persists
> because step_over_gc_barrier() remained unimplemented. (Duh!)
> 
> http://cr.openjdk.java.net/~rkennke/fixlockelimination-shjdk11/webrev.00/

(facepalms)

Looks good!

-Aleksey