Merged r19535 into 3.23 branch.

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

BUG=345715
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/182993004

git-svn-id: https://v8.googlecode.com/svn/branches/3.23@19574 ce2b1a6d-e550-0410-aec6-3dcde31c8c00