What is going on is that in 'update_costs_from_allocno' we try to
identify the smallest mode using narrower_subreg_mode to then update the
costs.

The two modes involved here are E_DImode and E_VNx2QImode, cause these
are not ordered we ICE in 'paradoxical_subreg_p'.

Now I don't know if the strategy we want is:

- In 'update_costs_from_allocno' when modes are not ordered instead of
  calling 'narrower_subreg_mode' just keep the current one.

- Always select the cheapest mode in terms of cost.

The attached I'm testing implements the second.

  Andrea