RFR: Extract Shenandoah superword check into ShenandoahBarrierSetC2

[Roland Westrelin]

> 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/

Let me take another look at this one. I have to figure out why it's
needed again. I thought some more about whether this could be upstreamed
even though it's shenandoah and I may give it a try.

Roland/