Use a special slot for HContext, and fetch the value from there each time it is used.  Allocate space for special slots in every HEnvironment.  Fill them with constant undefined.  Do not copy them to LEnvironment.

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Review URL: http://codereview.chromium.org/6929066

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7807 ce2b1a6d-e550-0410-aec6-3dcde31c8c00