[CostModel] Return Invalid cost in getArithmeticCost instead of crashing for scalable vectors.

This fixes an issue in BasicTTIImpl.h where it tries to do a
cast<FixedVectorType> on a scalable vector type in order to get the
scalarization cost. Because scalarization of scalable vectors is not
supported, we return Invalid instead.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D103798