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

[via llvm-commits]

================
@@ -2668,16 +2668,15 @@ void VPReplicateRecipe::execute(VPTransformState &State) {
     return;
   }
 
-  if (IsUniform) {
+  if (IsSingleScalar) {
     // Uniform within VL means we need to generate lane 0.
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ayalz wrote:

```suggestion
    // Single scalar across VL means we need to generate lane 0.
```

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