Debug-mode check added: optimized keyed store of a smi on x64 is done to a entry containing a smi.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/167063002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19389 ce2b1a6d-e550-0410-aec6-3dcde31c8c00