The Hydrogen graph translation does not build a branch for unary negation in
an effect context, so the baseline compiler should not do so either.

Review URL: http://codereview.chromium.org/6546050

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6871 ce2b1a6d-e550-0410-aec6-3dcde31c8c00