RFR: Traversal GC: query garbage counter only for regular regions

[Roman Kennke]

I found this in my patch queue from before FOSDEM. Back then I tripped
an assert about miscounted liveness data, which was caused by querying
garbage() on a humongous region. Re-ordering the calls fixes it. I
also made the assert a little weaker to not fire on humongous regions.

http://cr.openjdk.java.net/~rkennke/trav-humongous-live-data/webrev.00/

Passes: hotspot_gc_shenandoah