[CodeGen] Fix warnings in getCopyToPartsVector

Whilst trying to assemble the following test:

  clang/test/CodeGen/aarch64-sve-intrinsics/acle_sve_set2.c

I discovered we were hitting some warnings about possible invalid
calls to getVectorNumElements() in getCopyToPartsVector(). I've
tried to fix these by using ElementCount types where possible and
I've made the assumption that we don't support using a fixed width
vector to copy parts of a scalable vector, and vice versa. Looking
at how the copy is implemented I think that's the right thing for
now.

Differential Revision: https://reviews.llvm.org/D82744