Sign extend the dehoisted key at the definition point for x64 port only; for x32 port, we need to sign extend the dehoisted key at the use points.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/328553005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21806 ce2b1a6d-e550-0410-aec6-3dcde31c8c00