[NFCI][SimplifyCFG] Rewrite createUnreachableSwitchDefault()

The only thing that function should do as per it's semantic,
is to ensure that the switch's default is a block consisting only of
an `unreachable` terminator.

So let's just create such a block and update switch's default
to point to it. There should be no need for all this weird dance
around predecessors/successors.

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -4747,23 +4747,20 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
                                            DomTreeUpdater *DTU) {
   LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
   auto *BB = Switch->getParent();
-  BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
-      Switch->getDefaultDest(), Switch->getParent(), "", DTU);
   auto *OrigDefaultBlock = Switch->getDefaultDest();
+  OrigDefaultBlock->removePredecessor(BB);
+  BasicBlock *NewDefaultBlock = BasicBlock::Create(
+      BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
+      OrigDefaultBlock);
+  new UnreachableInst(Switch->getContext(), NewDefaultBlock);
   Switch->setDefaultDest(&*NewDefaultBlock);
-  if (DTU)
-    DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
-                       {DominatorTree::Delete, BB, OrigDefaultBlock}});
-  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
-  SmallVector<DominatorTree::UpdateType, 2> Updates;
-  if (DTU)
-    for (auto *Successor : successors(NewDefaultBlock))
-      Updates.push_back({DominatorTree::Delete, NewDefaultBlock, Successor});
-  auto *NewTerminator = NewDefaultBlock->getTerminator();
-  new UnreachableInst(Switch->getContext(), NewTerminator);
-  EraseTerminatorAndDCECond(NewTerminator);
-  if (DTU)
+  if (DTU) {
+    SmallVector<DominatorTree::UpdateType, 2> Updates;
+    Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
+    if (!is_contained(successors(BB), OrigDefaultBlock))
+      Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
     DTU->applyUpdates(Updates);
+  }
 }
 
 /// Turn a switch with two reachable destinations into an integer range

llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll

@@ -33,7 +33,7 @@ default:
 
 define void @test2(i2 %a) {
 ; CHECK-LABEL: @test2(
-; CHECK-NEXT:    switch i2 [[A:%.*]], label [[DEFAULT1:%.*]] [
+; CHECK-NEXT:    switch i2 [[A:%.*]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
 ; CHECK-NEXT:    i2 0, label [[CASE0:%.*]]
 ; CHECK-NEXT:    i2 1, label [[CASE1:%.*]]
 ; CHECK-NEXT:    i2 -2, label [[CASE2:%.*]]
@@ -53,7 +53,7 @@ define void @test2(i2 %a) {
 ; CHECK:       case3:
 ; CHECK-NEXT:    call void @foo(i32 3)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
-; CHECK:       default1:
+; CHECK:       .unreachabledefault:
 ; CHECK-NEXT:    unreachable
 ;
   switch i2 %a, label %default [i2 0, label %case0