[llvm] [VPlan] Use VPlan predecessors in VPWidenPHIRecipe (NFC). (PR #126388)

[via llvm-commits]

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@@ -136,19 +136,22 @@ void PlainCFGBuilder::fixPhiNodes() {
       // predecessor is the first operand of the recipe.
       assert(Phi->getNumOperands() == 2);
       BasicBlock *LoopPred = L->getLoopPredecessor();
-      VPPhi->addIncoming(
-          getOrCreateVPOperand(Phi->getIncomingValueForBlock(LoopPred)),
-          BB2VPBB[LoopPred]);
+      VPPhi->addOperand(
+          getOrCreateVPOperand(Phi->getIncomingValueForBlock(LoopPred)));
       BasicBlock *LoopLatch = L->getLoopLatch();
-      VPPhi->addIncoming(
-          getOrCreateVPOperand(Phi->getIncomingValueForBlock(LoopLatch)),
-          BB2VPBB[LoopLatch]);
+      VPPhi->addOperand(
+          getOrCreateVPOperand(Phi->getIncomingValueForBlock(LoopLatch)));
       continue;
     }
 
-    for (unsigned I = 0; I != Phi->getNumOperands(); ++I)
-      VPPhi->addIncoming(getOrCreateVPOperand(Phi->getIncomingValue(I)),
-                         BB2VPBB[Phi->getIncomingBlock(I)]);
+    // Add operands for VPPhi in the order matching its predecessors in VPlan.
+    DenseMap<const VPBasicBlock *, VPValue *> IncomingValues;
+    for (unsigned I = 0; I != Phi->getNumOperands(); ++I) {
+      IncomingValues[BB2VPBB[Phi->getIncomingBlock(I)]] =
+          getOrCreateVPOperand(Phi->getIncomingValue(I));
+    }
+    for (VPBlockBase *Pred : VPPhi->getParent()->getPredecessors())
+      VPPhi->addOperand(IncomingValues.lookup(Pred->getExitingBasicBlock()));
----------------
ayalz wrote:

Note: if/when the reverse mapping VPBB2BB is available, this could be done by
```
  VPPhi->addOperand(Phi->getIncomingValueForBlock(VPBB2BB[Pred->getExitingBasicBlock()]));
```

https://github.com/llvm/llvm-project/pull/126388