[VPlan] Remove redundant assignment in VPReductionPHIRecipe (NFC)

Suggested post-commit for 0e528ac404e13ed2d952a2d83aaf8383293c851e.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3413,7 +3413,7 @@ void VPReductionPHIRecipe::execute(VPTransformState &State) {
     // TODO: The sentinel value is not always necessary. When the start value is
     // a constant, and smaller than the start value of the induction variable,
     // the start value can be directly used to initialize the reduction phi.
-    StartV = Iden = StartV;
+    Iden = StartV;
     if (!ScalarPHI) {
       IRBuilderBase::InsertPointGuard IPBuilder(Builder);
       Builder.SetInsertPoint(VectorPH->getTerminator());