When the stub return a heap number we do a state transition to
a version HEAP_NUMBER that can handle -0.

There is room for further improvement in the typefeedback for the
case of -0. This change however does not address this and only fixes
the acute issue.
Review URL: http://codereview.chromium.org/7037025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7944 ce2b1a6d-e550-0410-aec6-3dcde31c8c00