[llvm] [VPlan] Rename isUniform(AfterVectorization) to isSingleScalar (NFC). (PR #140134)

[via llvm-commits]

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@@ -109,7 +109,7 @@ bool vputils::isUniformAcrossVFsAndUFs(VPValue *V) {
         // VPReplicateRecipe.IsUniform. They are also uniform across UF parts if
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ayalz wrote:

```suggestion
        // VPReplicateRecipe.IsSingleScalar. They are also uniform across UF parts if
```

https://github.com/llvm/llvm-project/pull/140134