to add/sub by doing the normal operation and then checking for overflow
afterwards. This generally relies on the DAG handling the later invalid
operations as well.
Fixes the 64-bit part of rdar://8622122 and rdar://8774702.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123908 91177308-0d34-0410-b5e6-96231b3b80d8
