RFR: Skip pre-evac verification and pacing on shortcut cycle path

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/verifier-shortcut/webrev.01/

This cleans up a corner case in our verification and pacing bindings: we don't need to call pacer on
shortcut path, and there is no need to do double verification on non-shortcut path. This also
naturally handles VerifyAfterGC on shortcut path.

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey