RFR (jdk11): Backport JDK-8217016 and JDK-8217874

[Roman Kennke]

This backports two long-standing changes from jdk12 -> sh/jdk11.

This is by far not a straight backport. It also ports over a bunch of 
machinery to generate cmpxchg and xchg. The change is fairly large, but 
on the plus side it *reverts* a bunch of shared-code changes vs. 
upstream. Overall, the involved Shenandoah code is now much closer to jdk12.

Backported changes:
- JDK-8217016: Shenandoah: Streamline generation of CAS barriers
https://bugs.openjdk.java.net/browse/JDK-8217016
- JDK-8217874: Shenandoah: AArch64: Clobbered register in 
ShenandoahBarrierSetAssembler::cmpxchg_oop()
https://bugs.openjdk.java.net/browse/JDK-8217874


Webrev:
http://cr.openjdk.java.net/~rkennke/backport-jdk11-2019-05-06/webrev.00/

Testing: hotspot_gc_shenandoah (x86/aarch64)

Good to push?

Roman