RFR(XS) Early return results imbalanced time partition

[Zhengyu Gu]

Imbalanced time partition results assertion failure in debug build.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/tp_unbalance/webrev.00/

Test:

   hotspot_gc_shenandoah: fastdebug


Thanks,

-Zhengyu