nir/spirv: do not require a format with images that are not sampled

As soon as we support shaderStorageImageWriteWithoutFormat we can see
write-only images (sampled == 2) that don't have a format specified.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/spirv/spirv_to_nir.c

@@ -874,8 +874,6 @@ vtn_handle_type(struct vtn_builder *b, SpvOp opcode,
          val->type->type = glsl_sampler_type(dim, is_shadow, is_array,
                                              glsl_get_base_type(sampled_type));
       } else if (sampled == 2) {
-         assert((dim == GLSL_SAMPLER_DIM_SUBPASS ||
-                 dim == GLSL_SAMPLER_DIM_SUBPASS_MS) || format);
          assert(!is_shadow);
          val->type->type = glsl_image_type(dim, is_array,
                                            glsl_get_base_type(sampled_type));