Ignore observed Double output in binary operations when all uses are truncating to Integer32

BUG=v8:2424

Review URL: https://codereview.chromium.org/14320021

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14381 ce2b1a6d-e550-0410-aec6-3dcde31c8c00