Fix the case when reordering shuffle and binop produces a constant.

This resolves PR19737.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208762 91177308-0d34-0410-b5e6-96231b3b80d8
Update of the documentation: I think we are now happy with Phabricator

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208764 91177308-0d34-0410-b5e6-96231b3b80d8