[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;
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ayalz wrote:

On second thought, could we agree on
```suggestion
    // Early exit operand should always be last, i.e., 0 if VPEarlyExitBlock has a single predecessor and 1 if it has two.
    unsigned EarlyExitIdx = ExitIRI->getNumOperands() - 1;
```

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