[AArch64][SVE] Optimize bitcasts between unpacked half/i16 vectors.

The case for nxv2f32/nxv2i32 was already covered by D104573.
This patch builds on top of that by making the mechanism work for
nxv2[b]f16/nxv2i16, nxv4[b]f16/nxv4i16 as well.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D106138