Revert "Add the nsw flag when we detect that an add will not signed overflow."

This reverts commit r210029.

It was not correctly handling cases where LHS and RHS had multiple but different
sign bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210048 91177308-0d34-0410-b5e6-96231b3b80d8
Convert test to FileCheck.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210049 91177308-0d34-0410-b5e6-96231b3b80d8