Inline number to string conversion for string addition into BinaryOp(Stub).

This fixes a performance regression that was caused by converting the
BinaryOpStub to a Hydrogen code stub. It also fixes a leftover TODO wrt.
the handling of Number*String or String*Number versions of the stub.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/27674002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17290 ce2b1a6d-e550-0410-aec6-3dcde31c8c00