RFR (S) 8135012: Don't use G1RootProcessor when scanning remembered sets

[Mikael Gerdin]

Hi all,

Please review this small refactoring in preparation of some further changes.

Calls to scan the remembered sets during a G1 evacuation are currently 
needlessly tunneled through G1RootProcessor before ending up in G1RemSet.

I suggest that we change the code to call oops_into_collection_set_do 
straight away and that G1CodeBlob is moved to its own file (which also 
makes it available from outside of G1RootProcessor).

Webrev: http://cr.openjdk.java.net/~mgerdin/8135012/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8135012
Testing: JPRT.

Thanks
/Mikael