[SVE] Fix implicit TypeSize->uint64_t conversion getCastInstrCost

In getCastInstrCost() when comparing different sizes for src and
dst types we should be using the TypeSize comparison operators
instead of relying upon TypeSize being converted a uin64_t.
Previously this meant we were dropping the scalable property and
treating fixed and scalable vector types the same.

Differential Revision: https://reviews.llvm.org/D83461