RFR: 8241153: Refactor HeapRegionManager::find_unavailable_from_idx to simplify expand_at

[Stefan Johansson]

Hi,

Please review this enhancement to simplify the expand code in G1.

Webrev: http://cr.openjdk.java.net/~sjohanss/8241153/00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8241153

Summary:
Use the bitmap search functions to find candidates in 
find_unavailable_from_idx and return a range to expand.

Testing:
Mach5 tier1-4

Thanks,
Stefan