[llvm] [VPlan] Don't reset canonical IV start value. (PR #161589)

[via llvm-commits]

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@@ -1234,6 +1234,17 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
   if (!Plan->isUnrolled())
     return;
 
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ayalz wrote:

```suggestion

  // Hoist an invariant increment Y of a canonical IV phi X, by having X start at Y. 
```

https://github.com/llvm/llvm-project/pull/161589