Before computing reaching definitions, the set of all definitions in a
function must be collected and they must be numbered.  Have the flow
graph builder collect definitions for stack-allocated variables into a
list, and implicitly number them with their list index.

Review URL: http://codereview.chromium.org/668257

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4064 ce2b1a6d-e550-0410-aec6-3dcde31c8c00