[SLP][REVEC] Fix cost model for getBuildVectorCost with FixedVectorType

ScalarTy.

BoUpSLP::gather always use CreateInsertVector for FixedVectorType
ScalarTy.

Author: HanKuanChen

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -9198,10 +9198,18 @@ class BoUpSLP::ShuffleCostEstimator : public BaseShuffleAnalysis {
       bool NeedShuffle =
           count(VL, *It) > 1 &&
           (VL.front() != *It || !all_of(VL.drop_front(), IsaPred<UndefValue>));
-      if (!NeedShuffle)
+      if (!NeedShuffle) {
+        if (isa<FixedVectorType>(ScalarTy)) {
+          assert(SLPReVec && "FixedVectorType is not expected.");
+          return TTI.getShuffleCost(
+              TTI::SK_InsertSubvector, VecTy, {}, CostKind,
+              std::distance(VL.begin(), It) * getNumElements(ScalarTy),
+              cast<FixedVectorType>(ScalarTy));
+        }
         return TTI.getVectorInstrCost(Instruction::InsertElement, VecTy,
                                       CostKind, std::distance(VL.begin(), It),
                                       PoisonValue::get(VecTy), *It);
+      }
 
       SmallVector<int> ShuffleMask(VL.size(), PoisonMaskElem);
       transform(VL, ShuffleMask.begin(), [](Value *V) {