[SVE][CodeGen] Bail out for scalable vectors in AArch64TargetLowering::ReconstructShuffle

Previously the code in AArch64TargetLowering::ReconstructShuffle assumed
the input vectors were always fixed-width, however this is not always
the case since you can extract elements from scalable vectors and insert
into fixed-width ones. We were hitting crashes here for two different
cases:

1. When lowering a fixed-length vector extract from a scalable vector
with i1 element types. This happens due to the fact the i1 elements
get promoted to larger integer types for fixed-width vectors and leads
to sequences of INSERT_VECTOR_ELT and EXTRACT_VECTOR_ELT nodes. In this
case AArch64TargetLowering::ReconstructShuffle will still fail to make
a transformation, but at least it no longer crashes.
2. When lowering a sequence of extractelement/insertelement operations
on mixed fixed-width/scalable vectors.

For now, I've just changed AArch64TargetLowering::ReconstructShuffle to
bail out if it finds a scalable vector.

Tests for both instances described above have been added here:

  (1) CodeGen/AArch64/sve-extract-fixed-vector.ll
  (2) CodeGen/AArch64/sve-fixed-length-reshuffle.ll

Differential Revision: https://reviews.llvm.org/D116602