[llvm] [VPlan] Replace RdxDesc with RecurKind in VPReductionPHIRecipe (NFC). (PR #142322)

[via llvm-commits]

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@@ -751,8 +751,7 @@ Value *VPInstruction::generate(VPTransformState &State) {
     // and will be removed by breaking up the recipe further.
     auto *PhiR = cast<VPReductionPHIRecipe>(getOperand(0));
     // Get its reduction variable descriptor.
-    const RecurrenceDescriptor &RdxDesc = PhiR->getRecurrenceDescriptor();
-    RecurKind RK = RdxDesc.getRecurrenceKind();
+    [[maybe_unused]] RecurKind RK = PhiR->getRecurrenceKind();
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ayalz wrote:

```suggestion
    RecurKind RK = PhiR->getRecurrenceKind();
```
current RK is set w/o `[[maybe_unused]]`, surely used to set IsSigned below?

https://github.com/llvm/llvm-project/pull/142322