RFR: Optimize CAS-null-obj for narrow-oops

[Roman Kennke]

For consistency with uncompressed oops, and as slight optimization,
avoid Shenandoah CAS-loop for narrow-oop-CASes with expected==NULL.

http://cr.openjdk.java.net/~rkennke/cas-narrow-null/webrev.00/

Tests: tier3_gc_shenandoah, some specjbb point tests

Ok?

Roman