RFR: JDK-8240315: Shenandoah: Rename ShLBN::get_barrier_strength()

[Roman Kennke]

I would like to rename
ShenandoahLoadReferenceBarrierNode::get_barrier_strength() to
is_lrb_useful(). See:

Bug:
https://bugs.openjdk.java.net/browse/JDK-8240315
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8240315/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman