Shenandoah: Rename ShLBN::get_barrier_strength()

ShenandoahLoadReferenceBarrier::get_barrier_strength() is named strangely. This stems from the original attempt to emit different-strength barriers for LRBs that only feed into load-likes (i.e. roughly like previous read barriers) and LRBs that also feed into store-likes (i.e. roughly like previous write-barriers). However, we never followed through with that, and probably never will because the small gains - if any - don't justify the large complexity (e.g. memory/ordering dependencies between weak and strong LRBs).

The actual implementation only has two strengths: strong and none. Strong means, it has a significant use, and none means no use. Let's rename it to is_lrb_useful().