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

unittests/ADT/ilistTest.cpp

@@ -0,0 +1,39 @@
+//===- llvm/unittest/ADT/APInt.cpp - APInt unit tests ---------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <ostream>
+#include "gtest/gtest.h"
+#include "llvm/ADT/ilist.h"
+#include "llvm/ADT/ilist_node.h"
+
+using namespace llvm;
+
+namespace {
+
+struct Node : ilist_node<Node> {
+  int Value;
+
+  Node() {}
+  Node(int _Value) : Value(_Value) {}
+};
+
+TEST(ilistTest, Basic) {
+  ilist<Node> List;
+  List.push_back(Node(1));
+  EXPECT_EQ(1, List.back().Value);
+  EXPECT_EQ(0, List.back().getPrevNode());
+  EXPECT_EQ(0, List.back().getNextNode());
+
+  List.push_back(Node(2));
+  EXPECT_EQ(2, List.back().Value);
+  EXPECT_EQ(2, List.front().getNextNode()->Value);
+  EXPECT_EQ(1, List.back().getPrevNode()->Value);
+}
+
+}