[NFC][LowerMemIntrinsics] Use TypeSize consistently for type sizes (#179945)
PR #169040 already started using `TypeSize` for the return value of
`DataLayout::getType*Size` in the memset lowering, this PR adjusts other uses
in LowerMemIntrinsics to do the same. Currently, scalable vector types are not
supported as access types for the mem-intrinsic lowering.