HPhis with HParameter inputs must have Tagged representation

BUG=v8:3670
LOG=n
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/702053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25153 ce2b1a6d-e550-0410-aec6-3dcde31c8c00