[SandboxVec][DAG][NFC] Refactor setNextNode() and setPrevNode() (#122363)

This patch updates DAG's `setNextNode()` and `setPrevNode()` to update
both nodes of the link.

Author: vporpo

llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.h

@@ -216,9 +216,18 @@ class MemDGNode final : public DGNode {
   /// Memory predecessors.
   DenseSet<MemDGNode *> MemPreds;
   friend class PredIterator; // For MemPreds.
-
-  void setNextNode(MemDGNode *N) { NextMemN = N; }
-  void setPrevNode(MemDGNode *N) { PrevMemN = N; }
+  /// Creates both edges: this<->N.
+  void setNextNode(MemDGNode *N) {
+    NextMemN = N;
+    if (NextMemN != nullptr)
+      NextMemN->PrevMemN = this;
+  }
+  /// Creates both edges: N<->this.
+  void setPrevNode(MemDGNode *N) {
+    PrevMemN = N;
+    if (PrevMemN != nullptr)
+      PrevMemN->NextMemN = this;
+  }
   friend class DependencyGraph; // For setNextNode(), setPrevNode().
   void detachFromChain() {
     if (PrevMemN != nullptr)

llvm/lib/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.cpp

@@ -283,8 +283,6 @@ void DependencyGraph::createNewNodes(const Interval<Instruction> &NewInterval) {
     // Build the Mem node chain.
     if (auto *MemN = dyn_cast<MemDGNode>(N)) {
       MemN->setPrevNode(LastMemN);
-      if (LastMemN != nullptr)
-        LastMemN->setNextNode(MemN);
       LastMemN = MemN;
     }
   }
@@ -302,7 +300,6 @@ void DependencyGraph::createNewNodes(const Interval<Instruction> &NewInterval) {
            "Wrong order!");
     if (LinkTopN != nullptr && LinkBotN != nullptr) {
       LinkTopN->setNextNode(LinkBotN);
-      LinkBotN->setPrevNode(LinkTopN);
     }
 #ifndef NDEBUG
     // TODO: Remove this once we've done enough testing.
@@ -394,22 +391,14 @@ void DependencyGraph::notifyMoveInstr(Instruction *I, const BBIterator &To) {
   if (To != BB->end()) {
     DGNode *ToN = getNodeOrNull(&*To);
     if (ToN != nullptr) {
-      MemDGNode *PrevMemN = getMemDGNodeBefore(ToN, /*IncludingN=*/false);
-      MemDGNode *NextMemN = getMemDGNodeAfter(ToN, /*IncludingN=*/true);
-      MemN->PrevMemN = PrevMemN;
-      if (PrevMemN != nullptr)
-        PrevMemN->NextMemN = MemN;
-      MemN->NextMemN = NextMemN;
-      if (NextMemN != nullptr)
-        NextMemN->PrevMemN = MemN;
+      MemN->setPrevNode(getMemDGNodeBefore(ToN, /*IncludingN=*/false));
+      MemN->setNextNode(getMemDGNodeAfter(ToN, /*IncludingN=*/true));
     }
   } else {
     // MemN becomes the last instruction in the BB.
     auto *TermN = getNodeOrNull(BB->getTerminator());
     if (TermN != nullptr) {
-      MemDGNode *PrevMemN = getMemDGNodeBefore(TermN, /*IncludingN=*/false);
-      PrevMemN->NextMemN = MemN;
-      MemN->PrevMemN = PrevMemN;
+      MemN->setPrevNode(getMemDGNodeBefore(TermN, /*IncludingN=*/false));
     } else {
       // The terminator is outside the DAG interval so do nothing.
     }