Merging r168181: into 3.2 release branch.

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.

llvm-svn: 168447