RFR (S): Move shenandoah oop CAS to MacroAssembler

[Roland Westrelin]

> Please review this change, which moves OOP CAS from .ad to
> MacroAssembler, documents, and optimizes it a little:
>   http://cr.openjdk.java.net/~shade/shenandoah/c2-cas-macroasm/webrev.01/

That looks good to me.

Roland.