Next bunch of fixes for check elimination.

- Canonicalize HCheckMapValue with constant map to
  HCheckMaps, and get rid of the special treatment
  during check elimination.
- Track only stable object maps for HConstants and
  add CHECK()s to verify state during code generation.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/263923004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21154 ce2b1a6d-e550-0410-aec6-3dcde31c8c00