RFR: Cherry-pick: 8213199: GC abstraction for Assembler::needs_explicit_null_check() plus Shenandoah parts

[Roman Kennke]

In preparation for next upstream merge (reduce conflicts and headaches
sorting it out), this cherry picks "8213199: GC abstraction for
Assembler::needs_explicit_null_check()" plus the corresponding Zero
fixes by Aleksey (JDK-8213711) and adds the Shenandoah parts.

http://cr.openjdk.java.net/~rkennke/JDK-8213199-shenandoah/webrev.00/

Testing: tier3_gc_shenandoah

Ok?

Roman