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

[Roman Kennke]

ok. Thanks!

Roman


> 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}
>