ARM only: Check for -0 only when really necessary. Tiny code de-duplication.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/14499011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14479 ce2b1a6d-e550-0410-aec6-3dcde31c8c00