[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[David Sherwood via llvm-commits]

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@@ -10365,24 +10385,7 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
   VPBasicBlock *Header = VectorLoop->getEntryBasicBlock();
   Header->setName("vec.epilog.vector.body");
 
-  // Re-use the trip count and steps expanded for the main loop, as
----------------
david-arm wrote:

I guess in theory this code could be moved to the end of the function in a separate NFC patch, with this patch then adding the new bits:

```
    auto *VPI = dyn_cast<VPInstruction>(&R);
    if (VPI) {
      VPI->replaceAllUsesWith(Plan.getOrAddLiveIn(
          ToFrozen[VPI->getOperand(0)->getLiveInIRValue()]));
      continue;
    }
```

However, I don't want to create unnecessary burden as I know this is an important fix. I'll leave it up to you!

https://github.com/llvm/llvm-project/pull/132691