RFR: unsafe oop comparison

[Zhengyu Gu]

Crash with -XX:+VerifyStrictOopOperations -XX:+PrintAssembly in debug build.

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

Test:

hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu