RFR: Move shenandoah_eliminate_wb_pre to ShenandoahBarrierSetC2

[Roman Kennke]

Compile::shenandoah_eliminate_wb_pre() can be moved into proper GC
interface in ShenandoahBarrierSetC2.

It appears that this does an extra round of eliminate_gc_barrier() in
PhaseMacroExpand::process_users_of_allocation(). It doesn't seem to
hurt. Other than that, the code should be equivalent.

http://cr.openjdk.java.net/~rkennke/c2-move-eliminate/webrev.00/

Testing: tier3_gc_shenandoah

Roman