[llvm] [VPlan] Make canonical IV part of the region (PR #156262)

[via llvm-commits]

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@@ -9056,15 +9047,27 @@ static SmallVector<Instruction *> preparePlanForEpilogueVectorLoop(
          "ScalarIVSteps when resetting the start value");
   VPBuilder Builder(Header, Header->getFirstNonPhi());
   VPInstruction *Add = Builder.createAdd(IV, VPV);
-  IV->replaceAllUsesWith(Add);
-  Add->setOperand(0, IV);
+  // Replace all users of the canonical IV with the offset version, except for
+  // the Add itself and the canonical IV increment.
+  auto *Increment = vputils::findCanonicalIVIncrement(Plan);
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ayalz wrote:

Worth asserting here that Increment != nullptr, as expected later.

https://github.com/llvm/llvm-project/pull/156262