Each context in the context chain has a corresponding hydrogen value.
The context values are used for global object lookup and context slot
lookup.  Add simple (non-compound) assignment to context slots.

Review URL: http://codereview.chromium.org/6390003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6615 ce2b1a6d-e550-0410-aec6-3dcde31c8c00