InstCombine: Annotate sub with nuw when we prove it's safe

We can prove that a 'sub' can be a 'sub nuw' if the left-hand side is
negative and the right-hand side is non-negative.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216045 91177308-0d34-0410-b5e6-96231b3b80d8