Fix off-by-one error in AstTyper.

This causes the first parameter to be confused with the first
stack local when we collect type information.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/105943007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18296 ce2b1a6d-e550-0410-aec6-3dcde31c8c00