We cannot add new JSToNumber nodes here in general, because:

 a) The inserted ToNumber operation screws up observability of valueOf.
 b) Deoptimization at ToNumber doesn't have corresponding bailout id.

TEST=cctest,mjsunit
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/649543004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24660 ce2b1a6d-e550-0410-aec6-3dcde31c8c00