nir/deref: Skip over casts in fixup_deref_modes

nir/deref: Skip over casts in fixup_deref_modes

This pass is used when, for instance, we lazily change the mode of
variables rather than replacing the variable with a new one.  Since we
only do this in cases where we know we have full deref chains, it's ok
to just skip them in fixup_deref_modes.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>