RFR (S): Move shenandoah oop CAS to MacroAssembler

[Aleksey Shipilev]

Hi,

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/

Testing: hotspot/gc/shenandoah, jcstress (test-custom)

Thanks,
-Aleksey