Fix invalid type assumption in GenerateNumberToString().

We don't have any type feedback in %_NumberToString(), so it's
a bug to assume Type::Number here.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/143893002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18704 ce2b1a6d-e550-0410-aec6-3dcde31c8c00