Fix and cleanup Map::GeneralizeRepresentation().

TEST=mjsunit/regress/regress-365172-[1-3],mjsunit/field-type-tracking
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/259993004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21006 ce2b1a6d-e550-0410-aec6-3dcde31c8c00