Reduce usage of AssumeRepresentation

BUG=
R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16871011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15132 ce2b1a6d-e550-0410-aec6-3dcde31c8c00