[llvm] [VPlan] Add exit phi operands during initial construction (NFC). (PR #136455)

[via llvm-commits]

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@@ -2505,35 +2505,43 @@ void VPlanTransforms::handleUncountableEarlyExit(
   VPBuilder EarlyExitB(VectorEarlyExitVPBB);
   for (VPRecipeBase &R : VPEarlyExitBlock->phis()) {
     auto *ExitIRI = cast<VPIRPhi>(&R);
-    PHINode &ExitPhi = ExitIRI->getIRPhi();
-    VPValue *IncomingFromEarlyExit = RecipeBuilder.getVPValueOrAddLiveIn(
-        ExitPhi.getIncomingValueForBlock(UncountableExitingBlock));
-
+    // By default, assume early exit operand is first, e.g., when the two exit
+    // blocks are distinct - VPEarlyExitBlock has a single predecessor.
+    unsigned EarlyExitIdx = 0;
     if (OrigLoop->getUniqueExitBlock()) {
+      // The incoming values currently correspond to the original IR
+      // predecessors. After the transform, the first incoming value is coming
+      // from the original loop latch, while the second operand is from the
+      // early exit. Swap the phi operands, if the first predecessor in the
+      // original IR is not the loop latch.
+      if (*pred_begin(VPEarlyExitBlock->getIRBasicBlock()) !=
+          OrigLoop->getLoopLatch())
+        ExitIRI->swapOperands();
+
+      EarlyExitIdx = 1;
       // If there's a unique exit block, VPEarlyExitBlock has 2 predecessors
-      // (MiddleVPBB and NewMiddle). Add the incoming value from MiddleVPBB
-      // which is coming from the original latch.
-      VPValue *IncomingFromLatch = RecipeBuilder.getVPValueOrAddLiveIn(
-          ExitPhi.getIncomingValueForBlock(OrigLoop->getLoopLatch()));
-      ExitIRI->addOperand(IncomingFromLatch);
-      ExitIRI->extractLastLaneOfOperand(MiddleBuilder);
+      // (MiddleVPBB and NewMiddle). Extract the last lane of the incoming value
+      // from MiddleVPBB which is coming from the original latch.
----------------
ayalz wrote:

```suggestion
      // The first of two operands corresponds to the latch exit, via MiddleVPBB predecessor.
      // Extract its last lane.
```
(The other immediate predecessor, which corresponds to early exit, is VectorEarlyExitVPBB, rather than NewMiddle). 

https://github.com/llvm/llvm-project/pull/136455