The Hydrogen environment only tracks values for stack-allocated variables.
It is a precondition of HEnvironment::Lookup(variable) that variable is
stack-allocated.  The check was missing at one call site.

Review URL: http://codereview.chromium.org/6034004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6094 ce2b1a6d-e550-0410-aec6-3dcde31c8c00