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

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

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.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D146118