Fix for a smi stores optimization on x64 with a test case.

BUG=338425
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/152923006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19288 ce2b1a6d-e550-0410-aec6-3dcde31c8c00