RFR: Minor cleanup, uses latest Atomic API

[Zhengyu Gu]

Minor cleanup. Uses Atomic::sub() and Atomic::replace_if_null() APIs.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/atomic_cleanup/webrev.00/


Test:
   hotspot_gc_shenandoah (fastdebug + release)


Thanks,

-Zhengyu