Change the HGraphBuilder to dispatch on the context.

Change the HGraphBuilder to dispatch on the context.

Before, expressions didn't take advantage of knowing their context in
the AST.  Now, we use the context to decide what to do with a value at
the end of visiting an expression.

Review URL: http://codereview.chromium.org/5620007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5954 ce2b1a6d-e550-0410-aec6-3dcde31c8c00