[llvm] [VPlan] Refactor VPlan creation, add transform introducing region (NFC). (PR #128419)

[via llvm-commits]

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@@ -3589,6 +3574,8 @@ class VPlan {
     TripCount = NewTripCount;
   }
 
+  void setTripCount(VPValue *NewTripCount) { TripCount = NewTripCount; }
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ayalz wrote:

Better place setTripCount() before resetTripCount().
```suggestion
  // Set the trip count assuming it is currently null; if it is not - use resetTripCount().
  void setTripCount(VPValue *NewTripCount) {
    assert(!TripCount && "TripCount expected to be null");
    TripCount = NewTripCount;
  }
```

https://github.com/llvm/llvm-project/pull/128419