[SVE] Fix incorrect DAG combines when extracting fixed-width from scalable vectors

We were previously silently generating incorrect code when extracting a
fixed-width vector from a scalable vector. This is worse than crashing,
since the user will have no indication that this is currently unsupported
behaviour. I have fixed the code to only perform DAG combines when safe
to do so, i.e. the input and output vectors are both fixed-width or
both scalable.

Test added here:

  CodeGen/AArch64/sve-extract-scalable-vector.ll

Differential revision: https://reviews.llvm.org/D110624