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

This reverts commit r209746.

It looks it is causing a crash while building libcxx. I am trying to get a
reduced testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209762 91177308-0d34-0410-b5e6-96231b3b80d8