[SLP] ExtractElement Cost Calculation

[bababuck]

I noticed we calculate the extraction cost differently in different places, I'm curious why we would want to use different models at these different times.

When we are trying to include an ExtractElement instruction in our Vector Chain (SLPVectorizer.cpp::14677). This was updated in #125725

    auto GetVectorCost = [&, &TTI = *TTI](InstructionCost CommonCost) {
      return CommonCost - (DemandedElts.isZero()
                               ? TTI::TCC_Free
                               : TTI.getScalarizationOverhead(
                                     SrcVecTy, DemandedElts, /*Insert=*/false,
                                     /*Extract=*/true, CostKind));
    };

But when we have an external use that will require an ExtractElement (SLPVectorizer.cpp::16305):

      ExtraCost =
          getVectorInstrCost(*TTI, ScalarTy, Instruction::ExtractElement, VecTy,
                             CostKind, EU.Lane, EU.Scalar, ScalarUserAndIdx);

For class TargetTransformInfoImplBase:

  virtual InstructionCost getScalarizationOverhead(
      VectorType *Ty, const APInt &DemandedElts, bool Insert, bool Extract,
      TTI::TargetCostKind CostKind, bool ForPoisonSrc = true,
      ArrayRef<Value *> VL = {}) const {
    return 0;
  }
...
  virtual InstructionCost getVectorInstrCost(unsigned Opcode, Type *Val,
                                             TTI::TargetCostKind CostKind,
                                             unsigned Index, const Value *Op0,
                                             const Value *Op1) const {
    return 1;
  }

[alexey-bataev]

Generally speaking, all getVectorInstrCost should be replaced by getScalarizationOverhead calls. Just need to implement grouping by the vector nodes.
PS. Do not look into base implementations, look into specific target implementations. Base implementation does not mean much in many cases