Moved the function GetValue from the code generator to the Reference

helper class.

As a consequence, also removed the optional Reference from the code
generator state and got rid of some state-dependent behavior in the
code generator.
Review URL: http://codereview.chromium.org/6538

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@466 ce2b1a6d-e550-0410-aec6-3dcde31c8c00