[SVE][CodeGen] Fix up warnings in sve-split-insert/extract tests

I have fixed up some more ElementCount/TypeSize related warnings in
the following tests:

  CodeGen/AArch64/sve-split-extract-elt.ll
  CodeGen/AArch64/sve-split-insert-elt.ll

In SelectionDAG::CreateStackTemporary we were relying upon the implicit
cast from TypeSize -> uint64_t when calling MachineFrameInfo::CreateStackObject.
I've fixed this by passing in the known minimum size instead, which I
believe is fine because the associated stack id indicates whether this
is a scalable object or not.

I've also fixed up a case in TargetLowering::SimplifyDemandedBits when
extracting a vector element from a scalable vector. The result is a scalar,
hence it wasn't caught at the start of the function. If the vector is
scalable we just bail out for now.

Differential Revision: https://reviews.llvm.org/D86431