AArch64: NEON loadV2 and storeV2 addressing is wrong

c2 loadV2 and storeV2 are used for Vector API mask value load/store with
size of halfword. Their load/store immediate offset should be similar
to other vector load/store, e.g. loadV4/storeV4, valid only for
unscaled signed offset of imm9 or scaled unsigned offset of imm12:scale.