[VPlan] Add insertOnEdge to VPBlockUtils (NFC).

Add a new helper to insert a new VPBlockBase on an edge between 2
blocks. Suggested in #114292
and also useful for some existing code.

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -4177,6 +4177,24 @@ class VPBlockUtils {
       return cast<BlockTy>(&Block);
     });
   }
+
+  /// Inserts \p BlockPtr on the edge between \p From and \p To. That is, update
+  /// \p From's successor to \p To to point to \p BlockPtr and \p To's
+  /// predecessor from \p From to \p BlockPtr. \p From and \p To are added to \p
+  /// BlockPtr's predecessors and successors respectively. There must be a
+  /// single edge between \p From and \p To.
+  static void insertOnEdge(VPBlockBase *From, VPBlockBase *To,
+                           VPBlockBase *BlockPtr) {
+    auto &Successors = From->getSuccessors();
+    auto &Predecessors = To->getPredecessors();
+    assert(count(Successors, To) == 1 && count(Predecessors, From) == 1 &&
+           "must have single between From and To");
+    unsigned SuccIdx = std::distance(Successors.begin(), find(Successors, To));
+    unsigned PredIx =
+        std::distance(Predecessors.begin(), find(Predecessors, From));
+    VPBlockUtils::connectBlocks(From, BlockPtr, -1, SuccIdx);
+    VPBlockUtils::connectBlocks(BlockPtr, To, PredIx, -1);
+  }
 };
 
 class VPInterleavedAccessInfo {

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -360,8 +360,7 @@ static void addReplicateRegions(VPlan &Plan) {
     // Record predicated instructions for above packing optimizations.
     VPBlockBase *Region = createReplicateRegion(RepR, Plan);
     Region->setParent(CurrentBlock->getParent());
-    VPBlockUtils::connectBlocks(CurrentBlock, Region, -1, 0);
-    VPBlockUtils::connectBlocks(Region, SplitBlock, 0, -1);
+    VPBlockUtils::insertOnEdge(CurrentBlock, SplitBlock, Region);
   }
 }