Drop unnecessary receiver validity checks from {Load,Store}IC_Normal.

Since these builtins are used as handlers after a map check/dispatch, they don't need to check the receiver again.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/390053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22391 ce2b1a6d-e550-0410-aec6-3dcde31c8c00