InstCombine: sub nsw %x, C -> add nsw %x, -C if C isn't INT_MIN

We can preserve nsw during this transform if -C won't overflow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216269 91177308-0d34-0410-b5e6-96231b3b80d8