This change add simple local live variable information to 
the fast code generator.  It supports only AST nodes that 
are accepted by the syntax checker.

Each variable use points to a variable definition structure
which contains the last use of the definition.

To determine whether a variable is live after a certain point
we can check whether its last use occurs later in the evaluation
order defined by the AST labeling number.

The new information is currently only printed out together with
the IR and not yet used for code generation.

Review URL: http://codereview.chromium.org/603004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3839 ce2b1a6d-e550-0410-aec6-3dcde31c8c00