RFR (S) 8229419: Shenandoah: Cleanup LRB strength selector code

[Aleksey Shipilev]

RFE:
 https://bugs.openjdk.java.net/browse/JDK-8229419
 https://cr.openjdk.java.net/~shade/8229419/webrev.01/

Current code in ShenandoahLoadReferenceBarrierNode::get_barrier_strength selects the LRB strength
based on observed nodes. It needs some cleanups:
 - No need for WEAK strength, as it is not handled anywhere
 - Default case need to print node name into failure message, so it is available in hs_err
 - Nodes should be grouped together by class

Testing: hotspot_gc_shenandoah {fastdebug, release}

-- 
Thanks,
-Aleksey