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

include/llvm/ADT/ilist_node.h

@@ -49,6 +49,56 @@ class ilist_node : private ilist_half_node<NodeTy> {
   void setNext(NodeTy *N) { Next = N; }
 protected:
   ilist_node() : Next(0) {}
+
+public:
+  /// @name Adjacent Node Accessors
+  /// @{
+
+  /// \brief Get the previous node, or 0 for the list head.
+  NodeTy *getPrevNode() {
+    NodeTy *Prev = this->getPrev();
+
+    // Check for sentinel.
+    if (!Prev->getNext())
+      return 0;
+
+    return Prev;
+  }
+
+  /// \brief Get the previous node, or 0 for the list head.
+  const NodeTy *getPrevNode() const {
+    NodeTy *Prev = this->getPrev();
+
+    // Check for sentinel.
+    if (!Prev->getNext())
+      return 0;
+
+    return Prev;
+  }
+
+  /// \brief Get the next node, or 0 for the list tail.
+  NodeTy *getNextNode() {
+    NodeTy *Next = getNext();
+
+    // Check for sentinel.
+    if (!Next->getNext())
+      return 0;
+
+    return Next;
+  }
+
+  /// \brief Get the next node, or 0 for the list tail.
+  const NodeTy *getNextNode() const {
+    NodeTy *Next = getNext();
+
+    // Check for sentinel.
+    if (!Next->getNext())
+      return 0;
+
+    return Next;
+  }
+
+  /// @}
 };
 
 } // End llvm namespace