Use ast_id (id) instead of return_id (LoadId) in simulates for load instructions.

R=svenpanne@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23889006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16519 ce2b1a6d-e550-0410-aec6-3dcde31c8c00