### Godot version

4.0b17

### System information

macOS 13.2

### Issue description

If you make a visual spatial shader and that uses an instance uniform and you have some nodes with previews (the little graphic view) for example like this:

![Screenshot 2023-02-02 at 18 54 15](https://user-images.githubusercontent.com/17763524/216403612-05591145-e32a-4b4b-a57f-9ebd47be4082.jpg)

You will get a strange error in the output whenever you load the shader like this:
```
--Main Shader--
    1 | shader_type canvas_item;
    2 | 
E   3-> instance uniform float InstanceParameter : hint_range(0, 1) = 1;
    4 | 
    5 | 
    6 | 
    7 | void fragment() {
    8 | 
    9 |  float n_out21p0 = InstanceParameter;
   10 | 
   11 | 
   12 | 
   13 |  float n_out14p0 = 1.0 - n_out21p0;
   14 | 
   15 | 
   16 |  COLOR.rgb = vec3(n_out14p0);
   17 | }
   18 | 
  :3 - Uniform instances are not yet implemented for 'canvas_item' shaders.
  Shader compilation failed.
  ./servers/rendering/renderer_rd/shader_rd.h:140 - Condition "!version" is true. Returning: RID()
```

After a lot of experimenting the reason for this seems to be that the visual shader editor is actually also generating canvas_item shaders to render the little node previews (an elegant solution!). And for this shader code it's using the uniforms the nodes are connected to. However, this doesn't seem to account for the fact that we now have instance uniforms that are supported in spatial visual shaders, but not in canvas_item shaders. So it's failing to compile the shaders for the previews and outputs this error in the process. The failures have probably been happening since instance uniforms were added, but the error reporting has been added more recently so I started seeing these in one of the later beta versions.

This doesn't seem to affect the generated shader performance, but it's outputting some confusing errors (took me a good while to figure out) and the previews for the nodes just show all white so you could say it affects the usability of the shader editor.

### Steps to reproduce

- Create a spatial visual shader
- Add an input parameter and set the qualifier to be "Instance"
- Link the parameter value to some node with a preview
- Link that node to the output
- Observe output for errors
- Open/close the file in the editor or open/close the preview on the node to see errors again.

### Minimal reproduction project

The shader from the example: [VisualShaderInstanceUniform.zip](https://www.lostminds.com/godot/VisualShaderInstanceUniform.zip)
void _delete_nodes(int p_type, const List<int> &p_nodes);
	void _delete_node_request(int p_type, int p_node);
	void _delete_nodes_request(const TypedArray<StringName> &p_nodes);
	void _node_changed(int p_id);
	void _edit_port_default_input(Object *p_button, int p_node, int p_port);
	void _port_edited(const StringName &p_property, const Variant &p_value, const String &p_field, bool p_changing);
	int to_node = -1;
	int to_slot = -1;
	int from_node = -1;
	int from_slot = -1;
	HashSet<int> selected_constants;
	HashSet<int> selected_parameters;
	int selected_comment = -1;
	int selected_float_constant = -1;
	void _convert_constants_to_parameters(bool p_vice_versa);
	void _replace_node(VisualShader::Type p_type_id, int p_node_id, const StringName &p_from, const StringName &p_to);
	void _update_constant(VisualShader::Type p_type_id, int p_node_id, Variant p_var, int p_preview_port);
	void _update_parameter(VisualShader::Type p_type_id, int p_node_id, Variant p_var, int p_preview_port);
	void _connection_to_empty(const String &p_from, int p_from_slot, const Vector2 &p_release_position);
	void _connection_from_empty(const String &p_to, int p_to_slot, const Vector2 &p_release_position);
	void _comment_title_popup_show(const Point2 &p_position, int p_node_id);
	void _comment_title_popup_hide();
	void _comment_title_popup_focus_out();
	void _comment_title_text_changed(const String &p_new_text);
	void _comment_title_text_submitted(const String &p_new_text);
	void _comment_desc_popup_show(const Point2 &p_position, int p_node_id);
	void _comment_desc_popup_hide();
	void _comment_desc_confirm();
	void _comment_desc_text_changed();
	void _parameter_line_edit_changed(const String &p_text, int p_node_id);