RFR: Fix CAS-obj predicates and add expected-null-versions for cmpxchg-narrow-oop

[Roman Kennke]

I did some experiments and found that the expected-null-optimization in
the .ad files never actually trigger. Roland's recent aarch64 patch
seems to solve it, and this is the same for x86:

http://cr.openjdk.java.net/~rkennke/fixcaspredicates/webrev.01/

It also implements the same optimization for narrow-oops.

Testing: tier3_gc_shenandoah, some manual tests and inspections to make
sure that matches actually trigger

Ok?