Fix JSBuiltinReducer to deal with non-JSFunction callees.

R=titzer@chromium.org
TEST=mozilla

Review URL: https://codereview.chromium.org/589573002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24097 ce2b1a6d-e550-0410-aec6-3dcde31c8c00