RFR: Apply ShenandoahEvacOOMScope only for evac-taking paths in ShenandoahBarrierSet

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/sbs-oomscope/webrev.01/

There is no need to penalize non-traversal GC with entering evac-scope, because those paths are not
evacuating anything. I inverted the nested branches to highlight that we only need to do this for
traversal paths.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey