Add static analysis to AST expressions that records whether a negative zero will be treated identically to a positive zero in the expression's context.  Use this flag to avoid some tests in inlined smi code.
Review URL: http://codereview.chromium.org/965001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4130 ce2b1a6d-e550-0410-aec6-3dcde31c8c00