[BasicTTI] Set scalarization cost of scalable vector casts to Invalid.

When BasicTTIImpl::getCastInstrCost can't determine the cost of a
vector cast operation when the types need legalization, it falls
back to calculating scalarization costs. Instead of crashing on
`cast<FixedVectorType>(DstVTy)` when the type is a scalable vector,
return an Invalid cost.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D106655