RFR: 8173764: Assert in G1 BOT is wrong

[Stefan Johansson]

Hi,

Please review this fix for:
https://bugs.openjdk.java.net/browse/JDK-8173764

Webrev:
http://cr.openjdk.java.net/~sjohanss/8173764/hotspot.00/

Summary:
The changed assert, asserted that there was an object at the start of 
the heap. What we want to assert is that no object span over region 
boundaries. That basically means that there is an object at the start of 
every region, with one exception, continues humongous regions. They need 
special handling since they don't contain an object at the start of the 
region.

Testing:
* JPRT
* RBT tier2 + tier3

Thanks,
Stefan