RFR (11): Backport LRB-related fixes

[Roman Kennke]

This backports the bunch of bugfixes related to LRB, to sh/jdk11:

- JDK-8222129: Shenandoah: Missing CompareAndSwapP/N case in
get_barrier_strength()
- JDK-8222786: Shenandoah get_barrier_strength should accept all shapes
of (Weak)CAS reference barriers
- JDK-8223448: Shenandoah disabled barriers blocks omit LRB
- JDK-8224667: Shenandoah: Post-LRB cleanup

http://cr.openjdk.java.net/~rkennke/backport-jdk11-lrb-fixes/webrev.00/

My plan is to push this together with the big LRB backport, once both
are reviewed.

Testing: hotspot_gc_shenandoah ok

Roman