RFR (S): 8214277: Use merged G1ArchiveRegionMap for open and closed archive heap regions

[Thomas Schatzl]

Hi all,

   could I have reviews for this small cleanup/simplification that 
merges the open and closed archive region map into a single one?

CR:
https://bugs.openjdk.java.net/browse/JDK-8214277
Webrev:
http://cr.openjdk.java.net/~tschatzl/8214277/webrev/
Testing:
hs-tier1-5 (almost done, no issues), local gc/g1 jtreg

Thanks,
   Thomas