Perf regression: changes in KeyedStoreIC introduced polymorphism.

When fixing bug 350884, I introduced an error that meant we went
polymorphic in KeyedStoreIC where we stayed monomorphic before. This
CL addresses the error, while preserving the bug fix for 350884.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/198793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19893 ce2b1a6d-e550-0410-aec6-3dcde31c8c00