Fix PR14361: wrong simplification of A+B==B+A. You may think that the old logic

Fix PR14361: wrong simplification of A+B==B+A.  You may think that the old logic
replaced by this patch is equivalent to the new logic, but you'd be wrong, and
that's exactly where the bug was.  There's a similar bug in instsimplify which
manifests itself as instsimplify failing to simplify this, rather than doing it
wrong, see next commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168181 91177308-0d34-0410-b5e6-96231b3b80d8