RFR (M): 8184346: Clean up G1CMBitmap

[Thomas Schatzl]

Hi all,

  Erik and Mikael had a look at it and suggested several further
cleanups, removing about 40 LOC. These included:

- instead of G1CMBitMapR0 use properly const'ified G1CMBitmaps
- change the _start and _word_size members into an equivalent MemRegion
- minor cleanups, removing obsolete asserts.

Webrevs:
http://cr.openjdk.java.net/~tschatzl/8184346/webrev.1_to_2/ (diff)
http://cr.openjdk.java.net/~tschatzl/8184346/webrev.2/ (full)
Testing:
jprt

Thanks,
  Thomas