Make stack_parameter_count a plain register.

R=rossberg@chromium.org, danno@chromium.org

Review URL: https://codereview.chromium.org/28993003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17275 ce2b1a6d-e550-0410-aec6-3dcde31c8c00