Fix missing Smi check in grow mode keyed stores.

R=danno@chromium.org
TEST=mjsunit/regress/regress-grow-store-smi-check

Review URL: https://codereview.chromium.org/14352011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14332 ce2b1a6d-e550-0410-aec6-3dcde31c8c00