nir/from_ssa: Don't rewrite derefs destinations to registers

We already call nir_rematerialize_derefs_in_use_blocks_impl prior to
calling nir_lower_ssa_defs_to_regs_block so the assertion that all deref
uses in the block should hold.  This fixes the following CTS test when
SPIR-V optimization recipe 1:

dEQP-VK.glsl.struct.local.loop_nested_struct_array_vertex

Fixes: 606eb56a "intel/nir: Only lower load/store derefs"
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
(cherry picked from commit 00f385e6)