nir/samplers: don't zero samplers_used/txf.

This allows this pass to be run multiple times and the results are
just or'ed together.

It fixes on test on llvmpipe nir, and regresses none.

Suggested by Kenneth

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/glsl/gl_nir_lower_samplers_as_deref.c

@@ -328,9 +328,6 @@ gl_nir_lower_samplers_as_deref(nir_shader *shader,
    state.remap_table = _mesa_hash_table_create(NULL, _mesa_key_hash_string,
                                                _mesa_key_string_equal);
 
-   shader->info.textures_used = 0;
-   shader->info.textures_used_by_txf = 0;
-
    nir_foreach_function(function, shader) {
       if (function->impl)
          progress |= lower_impl(function->impl, &state);