RFR: Extract Shenandoah superword check into ShenandoahBarrierSetC2

[Roman Kennke]

It's probably worth pointing out that I found a few of our tests trigger
the condition that we're checking in the test *and* don't fail in any
way. And our testsuite passes (including a few tests that trigger the
condition). Might be worth to investigate if we actually still need the
code at all?

Roman

> 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
>