ADT: Add ilist_node::get{Prev,Next}Node, which return the adjacent node or null.

ADT: Add ilist_node::get{Prev,Next}Node, which return the adjacent node or null.
 - This provides a convenient alternative to using something llvm::prior or
   manual iterator access, for example::

    if (T *Prev = foo->getPrevNode())
      ...

   instead of::

     iterator it(foo);
     if (it != begin()) {
       --it;
       ...
     }

 - Chris, please review.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103647 91177308-0d34-0410-b5e6-96231b3b80d8