Revert "[objc-arc] Track if we encountered an additive overflow while computing {TopDown,BottomUp}PathCounts and do nothing if it occured."

This reverts commit r187941.

The commit was passing on my os x box, but it is failing on some non-osx
platforms. I do not have time to look into it now, so I am reverting and will
recommit after I figure this out.