RFR: JDK-8222129: Shenandoah: Missing CompareAndSwapP/N case in get_barrier_strength()

[Roman Kennke]

 Missing case CompareAndSwapN/P in get_barrier_strength() trips as
assert in shenadoahSupport.cpp.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8222129
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8222129/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?
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