RFR(S): 8155943: Move G1Eden/SurvivorRegions into their own source files

[Mikael Gerdin]

Hi all,

Please review this small change to move the G1Eden/SurvivorRegions 
classes into their own source files. The new classes were introduced by 
JDK-8150721.


Bug: https://bugs.openjdk.java.net/browse/JDK-8155943
Webrev: http://cr.openjdk.java.net/~mgerdin/8155943/webrev.0/
Testing: Local build

Thanks
/Mikael