Revert "Revert "InstCombine: Improvement to check if signed addition overflows.""

This reverts commit r209762, bringing back r209746. It was not responsible for the libc++ build failure

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209776 91177308-0d34-0410-b5e6-96231b3b80d8