RFR: Extract Shenandoah superword check into ShenandoahBarrierSetC2

[Roman Kennke]

We have a check in superword.cpp to prevent superword optimization if we
find a WB between the pre- and main-loop. This seems to be a rare
situation. I'd:
- put it inside #if INCLUDE_SHENANDOAHGC
- and UseShenandoahGC
- and extract the actual body of code into ShenandoahBarrierSetC2.cpp

Not sure if it's worth to make a real abstraction for it. It seems quite
specific.

http://cr.openjdk.java.net/~rkennke/shenandoah-superword/webrev.00/

Testing: tier3_gc_shenandoah

Roman