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

This reverts commit r209776.

It was miscompiling llvm::SelectionDAGISel::MorphNode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209817 91177308-0d34-0410-b5e6-96231b3b80d8