[CodeGen] Fix warnings in sve-vector-splat.ll and sve-trunc.ll

This patch fixes all remaining warnings in:

  llvm/test/CodeGen/AArch64/sve-trunc.ll
  llvm/test/CodeGen/AArch64/sve-vector-splat.ll

I hit some warnings related to getCopyPartsToVector. I fixed two
issues:

1. In widenVectorToPartType() we assumed that we'd always be
using BUILD_VECTOR nodes to expand from one vector type to another,
which is incorrect for scalable vector types. I've fixed this for now
by simply bailing out immediately for scalable vectors.
2. In getCopyToPartsVector() I've changed the code to compare
the element counts of different types.

Differential Revision: https://reviews.llvm.org/D83028