[BasicTTIImpl] Fix getCastInstrCost for scalable vectors by querying for ElementCount.

This fixes an overly restrictive assumption that the vector is a FixedVectorType,
in code that tries to calculate the cost of a cast operation when splitting
a too-wide vector. The algorithm works the same for scalable vectors, so this
patch removes the cast<FixedVectorType>.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D96253