RFR: Cleanup ShenandoahBarrierSet::write_barrier

[Aleksey Shipilev]

There seems to be lots of single-use methods with weird names, related to
SBS::write_barrier. They look like historical artifacts, in attempt to make
interpreter code run faster? Nowadays it is cleaner to just inline all of them
into the SBS::write_barrier:
  http://cr.openjdk.java.net/~shade/shenandoah/sbs-wb-cleanup/webrev.01/

This also cleans up double asserts, keeping only the relevant ones, and makes a
single JRT entry, removing a duplicate one.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey