Port of r19288 to a64: Fix for smi stores optimization.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/159513004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19329 ce2b1a6d-e550-0410-aec6-3dcde31c8c00