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

[Roman Kennke]

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/

Testing: hotspot_gc_shenandoah, the whole lucene testsuite (that exposed
the failure)


Roman