[X86]add assert to confirm not-null ptr in getArithmeticReductionCost

[XinWang10]

Summary

For the function getArithmeticReductionCost, it receive a ptr and dereferce it without check,
It is called many times in getTypeBasedIntrinsicInstrCost, the ptr passed to it is inited
from line 1709.

From the code, we can not ensure the ptr VecOpTy is inited when Tys is empty or Tys[VecTyIndex]
is not a VectorType, so that the getArithmeticReductionCost will do an undefined behavior.

I add assert to it, found the ptr passed to it in llvm tests are all not nullptr, but I think the check is
still meaningful for us.

[RKSimon]

LGTM - a reduction cost is almost impossible without the type anyhow

llvm/include/llvm/CodeGen/BasicTTIImpl.h

2337

Maybe "Unknown reduction vector type" ?

[XinWang10]

• rebase and update assert msg

[XinWang10]

Thanks for your review!

llvm/include/llvm/CodeGen/BasicTTIImpl.h

InstructionCost getOrderedReductionCost(unsigned Opcode, VectorType *Ty,
     ArithCost *= VTy->getNumElements();
 
     return ExtractCost + ArithCost;
   }
 
   InstructionCost getArithmeticReductionCost(unsigned Opcode, VectorType *Ty,
                                              std::optional<FastMathFlags> FMF,
                                              TTI::TargetCostKind CostKind) {
+    assert(Ty && "Unknown reduction vector type");
     if (TTI::requiresOrderedReduction(FMF))
       return getOrderedReductionCost(Opcode, Ty, CostKind);
     return getTreeReductionCost(Opcode, Ty, CostKind);
   }
 
   /// Try to calculate op costs for min/max reduction operations.
   /// \param CondTy Conditional type for the Select instruction.
   InstructionCost getMinMaxReductionCost(VectorType *Ty, VectorType *CondTy,