Revert "Fix and cleanup Map::GeneralizeRepresentation()."

This reverts commit r21006 for performance regressions.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/259003002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21009 ce2b1a6d-e550-0410-aec6-3dcde31c8c00