The other operand might be minus zero, and -0 + 0 = +0

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/52173003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17432 ce2b1a6d-e550-0410-aec6-3dcde31c8c00