RFR: Extract Shenandoah superword check into ShenandoahBarrierSetC2

[Roland Westrelin]

> 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.

I ran a bunch of tests without that change and nothing broke. Let's
remove it. We can add it back if needed:

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

Roland.