[llvm] [VPlan] Construct initial once and pass clones to tryToBuildVPlan (NFC). (PR #141363)

[via llvm-commits]

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@@ -524,7 +524,8 @@ class LoopVectorizationPlanner {
   /// returned VPlan is valid for. If no VPlan can be built for the input range,
   /// set the largest included VF to the maximum VF for which no plan could be
   /// built.
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ayalz wrote:

Worth updating the documentation, say by adding something like:
```suggestion
  /// built. Each VPlan is built starting from a copy of \p InitialPlan, which is a plain CFG VPlan wrapping the original scalar loop.
```
?

https://github.com/llvm/llvm-project/pull/141363