Make the User::value_op_iterator a random access iterator. I had written
this code ages ago and lost track of it. Seems worth doing though --
this thing can get called from places that would benefit from knowing
that std::distance is O(1). Also add a very fledgeling unittest for
Users and make sure various aspects of this seem to work reasonably.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206453 91177308-0d34-0410-b5e6-96231b3b80d8