Update and extend context specialization.

* Add store-chain folding to context specialization.
* Replace all uses of context with constant and then use
  a visitor to find the places to optimize.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/440053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22993 ce2b1a6d-e550-0410-aec6-3dcde31c8c00