RFR (XS) 8225441: Cleanup ShenandoahHeap::atomic_compare_exchange_oop

[Roman Kennke]

We could also just use RawAccess<>::oop_atomic_cmpxchg() instead. That
does the same thing afaics. However, doing our owns is better
backportable (to 8). What do you think?

Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8225441
> 
> These helper functions can use smaller name and be static to boot.
> 
> Webrev:
>   http://cr.openjdk.java.net/~shade/8225441/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>