ADT: Add some missing coverage for iplist::splice

These splices are interesting because they involve swapping two nodes in
the same list.  There are two ways to do this.  Assuming:

    A -> B -> [Sentinel]

You can either:
- splice B before A, with:        L.splice(A,       L, B) or
- splice A before Sentinel, with: L.splice(L.end(), L, A) to create:

    B -> A -> [Sentinel]

These two swapping-splices are somewhat interesting corner cases for
maintaining the list invariants.  The tests pass even with my new ilist
implementation, but I had some doubts about the latter when I was
looking at weird UB effects.  Since I can't find equivalent explicit
test coverage elsewhere it seems prudent to commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278887 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/ADT/ilistTest.cpp

@@ -64,6 +64,38 @@ TEST(ilistTest, SpliceOne) {
   EXPECT_EQ(3, List.back().Value);
 }
 
+TEST(ilistTest, SpliceSwap) {
+  ilist<Node> L;
+  Node N0(0);
+  Node N1(1);
+  L.insert(L.end(), &N0);
+  L.insert(L.end(), &N1);
+  EXPECT_EQ(0, L.front().Value);
+  EXPECT_EQ(1, L.back().Value);
+
+  L.splice(L.begin(), L, ++L.begin());
+  EXPECT_EQ(1, L.front().Value);
+  EXPECT_EQ(0, L.back().Value);
+
+  L.clearAndLeakNodesUnsafely();
+}
+
+TEST(ilistTest, SpliceSwapOtherWay) {
+  ilist<Node> L;
+  Node N0(0);
+  Node N1(1);
+  L.insert(L.end(), &N0);
+  L.insert(L.end(), &N1);
+  EXPECT_EQ(0, L.front().Value);
+  EXPECT_EQ(1, L.back().Value);
+
+  L.splice(L.end(), L, L.begin());
+  EXPECT_EQ(1, L.front().Value);
+  EXPECT_EQ(0, L.back().Value);
+
+  L.clearAndLeakNodesUnsafely();
+}
+
 TEST(ilistTest, UnsafeClear) {
   ilist<Node> List;