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

[via llvm-commits]

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@@ -352,6 +352,22 @@ std::unique_ptr<VPlan> PlainCFGBuilder::buildPlainCFG(
   Plan->getEntry()->setOneSuccessor(getOrCreateVPBB(TheLoop->getHeader()));
   Plan->getEntry()->setPlan(&*Plan);
 
+  // Fix VPlan loop-closed-ssa exit phi's by adding incoming operands to the
+  // VPIRInstructions wrapping them.
+  // Note that the operand order may need adjusting when predecessors are added,
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ayalz wrote:

```suggestion
  // Note that the operand order corresponds to IR predecessor order, and may need adjusting when VPlan predecessors are added,
```

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