- set "can be minus zero" flag properly so minus-zero checks are skipped
- skip "integer result?" check in division code when uses are truncating
- drive-by cleanup: consolidated computation of kCanOverflow flag for Add/Sub into range inference phase

BUG=v8:2132
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/16741002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15060 ce2b1a6d-e550-0410-aec6-3dcde31c8c00