RFC: Pick up jdk-13+18 to sh/jdk

[Aleksey Shipilev]

We have not synced up sh/jdk for a while now. Let's pick up to the latest upstream tag, jdk-13+18.
Merge is trivial. This should fix jcstress tests in sh/jdk (missing cases in get_barrier_strength).

Changesets:
  http://cr.openjdk.java.net/~shade/shenandoah/merges/jdk-13%2b18/changesets.txt

Testing: hotspot_gc_shenandoah {fastdebug, release}

-- 
Thanks,
-Aleksey