Correctly handle instances without elements in polymorphic keyed load/store.

BUG=331416
TEST=mjsunit/regress/regress-331416.js
LOG=Y
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/121893003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18484 ce2b1a6d-e550-0410-aec6-3dcde31c8c00