InstSimplify: Simplify (sub 0, X) -> X if it's NUW

This is a generalization of the X - (0 - Y) -> X transform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222611 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Propagate NSW for -X * -Y -> X * Y

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222612 91177308-0d34-0410-b5e6-96231b3b80d8