RFR(XS) A few of early returns still calling register_gc_end()

[Zhengyu Gu]

Hi,

I missed a few of early return cases, where still call 
register_gc_end(). Now ShenandoahGCSession's destructor should handle 
the cases.

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


Test:
hotspot_gc_shenandoah: fastdebug and release


Thanks,

-Zhengyu