RFR: Introduce assert_in_correct_region to verify object is in correct region

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/assert-correct-region/webrev.01/

RTM folks report there is an assertion failure on in their systems, seems like humongous region was
reclaimed too soon. Unfortunately, the assert there is not very informative, and we should make it a
bit better.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey