RFR(XS): Fix live data accounting for humongous region

[Zhengyu Gu]

We count a humongous region's live data in humongous start region, which 
can well exceed the size of the region. When calculating the region's 
garbage (used - live data), can result scary numbers.

Also, consolidated logic for finding humongous start region.

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

Test:
   hotspot_gc_shenandoah: fastdebug and release

Thanks,

-Zhengyu