diff --git a/llvm/lib/Transforms/Vectorize/VPlanCFG.h b/llvm/lib/Transforms/Vectorize/VPlanCFG.h
--- a/llvm/lib/Transforms/Vectorize/VPlanCFG.h
+++ b/llvm/lib/Transforms/Vectorize/VPlanCFG.h
@@ -144,7 +144,8 @@
 /// entry node of VPRegionBlocks. Exit blocks of a region implicitly have their
 /// parent region's successors. This ensures all blocks in a region are visited
 /// before any blocks in a successor region when doing a reverse post-order
-// traversal of the graph.
+// traversal of the graph. Region blocks themselves are not directly connected
+// to their successors.
 template <typename BlockPtrTy>
 class VPAllSuccessorsIterator
     : public iterator_facade_base<VPAllSuccessorsIterator<BlockPtrTy>,
@@ -166,9 +167,8 @@
   /// both the const and non-const operator* implementations.
   template <typename T1> static T1 deref(T1 Block, unsigned SuccIdx) {
     if (auto *R = dyn_cast<VPRegionBlock>(Block)) {
-      if (SuccIdx == 0)
-        return R->getEntry();
-      SuccIdx--;
+      assert(SuccIdx == 0);
+      return R->getEntry();
     }
 
     // For exit blocks, use the next parent region with successors.
@@ -194,7 +194,7 @@
                                  : Block->getNumSuccessors();
 
     if (auto *R = dyn_cast<VPRegionBlock>(Block))
-      return {R, NumSuccessors + 1};
+      return {R, 1};
     return {Block, NumSuccessors};
   }