RFR: Don't check store value in Shenandoah C2 verifier

[Roman Kennke]

Since LRB, we don't need any store-value barriers anymore (except for
the enqueue-barriers in traversal, but I don't think we even check
that). The C2 verifier pass shouldn't check and complain about missing
barriers.

http://cr.openjdk.java.net/~rkennke/fixoptoverifier/webrev.00/

Testing: hotspot_gc_shenandoah

Good to push?

Roman