[VPlan] Add PredIdx and SuccIdx arguments to connectBlocks (NFC).

Add extra arguments to connectBlocks which allow selecting which
existing predecessor/successor to update. This avoids having to
disconnect blocks first unnecessarily.

Suggested in llvm#114292.

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -4113,17 +4113,29 @@ class VPBlockUtils {
     IfFalse->setParent(BlockPtr->getParent());
   }
 
-  /// Connect VPBlockBases \p From and \p To bi-directionally. Append \p To to
-  /// the successors of \p From and \p From to the predecessors of \p To. Both
-  /// VPBlockBases must have the same parent, which can be null. Both
-  /// VPBlockBases can be already connected to other VPBlockBases.
-  static void connectBlocks(VPBlockBase *From, VPBlockBase *To) {
+  /// Connect VPBlockBases \p From and \p To bi-directionally. If \p PredIdx is
+  /// -1, append \p From to the predecessors of \p To, otherwise set \p To's
+  /// predecessor at \p PredIdx to \p From. If \p SuccIdx is -1, append \p To to
+  /// the successors of \p From, otherwise set \p From's successor at \p SuccIdx
+  /// to \p To. Both VPBlockBases must have the same parent, which can be null.
+  /// Both VPBlockBases can be already connected to other VPBlockBases.
+  static void connectBlocks(VPBlockBase *From, VPBlockBase *To,
+                            unsigned PredIdx = -1u, unsigned SuccIdx = -1u) {
     assert((From->getParent() == To->getParent()) &&
            "Can't connect two block with different parents");
-    assert(From->getNumSuccessors() < 2 &&
+    assert((SuccIdx != -1u || From->getNumSuccessors() < 2) &&
            "Blocks can't have more than two successors.");
-    From->appendSuccessor(To);
-    To->appendPredecessor(From);
+    assert((PredIdx != -1u || To->getNumPredecessors() < 2) &&
+           "Blocks can't have more than two predecessors.");
+    if (SuccIdx == -1u)
+      From->appendSuccessor(To);
+    else
+      From->getSuccessors()[SuccIdx] = To;
+
+    if (PredIdx == -1u)
+      To->appendPredecessor(From);
+    else
+      To->getPredecessors()[PredIdx] = From;
   }
 
   /// Disconnect VPBlockBases \p From and \p To bi-directionally. Remove \p To

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -360,9 +360,8 @@ static void addReplicateRegions(VPlan &Plan) {
     // Record predicated instructions for above packing optimizations.
     VPBlockBase *Region = createReplicateRegion(RepR, Plan);
     Region->setParent(CurrentBlock->getParent());
-    VPBlockUtils::disconnectBlocks(CurrentBlock, SplitBlock);
-    VPBlockUtils::connectBlocks(CurrentBlock, Region);
-    VPBlockUtils::connectBlocks(Region, SplitBlock);
+    VPBlockUtils::connectBlocks(CurrentBlock, Region, -1, 0);
+    VPBlockUtils::connectBlocks(Region, SplitBlock, 0, -1);
   }
 }