RFR (S): 8193312: Rename VM_CGC_Operation to VM_G1ConcurrentGCOperation

[Thomas Schatzl]

Hi all,

  can I have reviews for this renaming of the VM operation classes in
G1 to something a bit more like the remaining code.

I kept the VM_ prefix only for continuity with the parent class.

CR:
https://bugs.openjdk.java.net/browse/JDK-8193312
Webrev:
http://cr.openjdk.java.net/~tschatzl/8193312/webrev/
Testing:
compilation on all platforms

Thanks,
  Thomas