RFR: Only include regular and humongous regions in traversal set

[Roman Kennke]

We see an occasional assert in HeapUncommit test with traversal
heuristics. The reason is that we put *all* regions into the traversal
set. Which is, of course, braindead. We only want to put regular+used
and humongous(+used) regions in traversal set.

http://cr.openjdk.java.net/~rkennke/fixtraversalset/webrev.00/

Testing: tier3_gc_shenandoah

Ok?

Roman