RFR: Streamline+cleanup ShenandoahBarrierSetC1 and related code

[Aleksey Shipilev]

On 09/26/2018 03:36 PM, Roman Kennke wrote:
> http://cr.openjdk.java.net/~rkennke/c1-bs-cleanups/webrev.00/

Looks okay to me. Would be better if Roland looked at this.

-Aleksey