[SVE][CodeGen] Fix TypeSize/ElementCount related warnings in sve-split-store.ll

I have fixed up a number of warnings resulting from TypeSize -> uint64_t
casts and calling getVectorNumElements() on scalable vector types. I
think most of the changes are fairly trivial except for those in
DAGTypeLegalizer::SplitVecRes_MSTORE I've tried to ensure we create
the MachineMemoryOperands in a sensible way for scalable vectors.

I have added a CHECK line to the following test:

 CodeGen/AArch64/sve-split-store.ll

that ensures no new warnings are added.

Differential Revision: https://reviews.llvm.org/D86928