RFR: Add humongous regions support to partial GC

[Roman Kennke]

This adds support for humongous regions as root regions in partial GC.

I extended the ShenandoahHeapRegion::oop_iterate() to iterate over a
subset of a humongous object array (bounded by the region). This way we
avoid scanning the whole humongous object if the matrix only gives us a
single region as root.

Tested using gc-bench/arrayfragger (which crashed before).

http://cr.openjdk.java.net/~rkennke/partial-humongous/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/partial-humongous/webrev.00/>

Roman