Add const to some member functions of SuccIterator.

The operator+() and operator-() do not change the member
variables of SuccIterator.  This CL will qualify the *this*
pointer with const.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201933 91177308-0d34-0410-b5e6-96231b3b80d8