[AArch64][SVE] Add DAG-Combine to push bitcasts from floating point loads after DUPLANE128

This patch lowers
  duplane128(insert_subvector(undef, bitcast(op(128bitsubvec)), 0), 0)
to
  bitcast(duplane128(insert_subvector(undef, op(128bitsubvec), 0), 0)).

This enables floating-point loads to match patterns added in
https://reviews.llvm.org/D130010

Differential Revision: https://reviews.llvm.org/D130013