Shenandoah: Cleanup LRB strength selector code

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