Also add backend flags that tell whether integer division/modulus is
generally safe, i.e. does not trap on overflow or divide by zero.

TEST=unittests
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/681133004

Cr-Commit-Position: refs/heads/master@{#24942}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24942 ce2b1a6d-e550-0410-aec6-3dcde31c8c00