### Godot version

Beta 17, master

### System information

Windows 11

### Issue description

Godot gets a hard crash while attempting to code-complete for member `const` of a class.

### Steps to reproduce

```GDScript
enum E { V1 }

const e := E
```

And then type a `.` after the last `E`, which triggers code hints/auto-completion for subscripts based on `E` and causes a hard crash.

Does not happen for `var e := E.V1`.

### Minimal reproduction project

See above.
reduce_expression(p_binary_op->left_operand);
	if (p_binary_op->operation == GDScriptParser::BinaryOpNode::OP_TYPE_TEST && p_binary_op->right_operand && p_binary_op->right_operand->type == GDScriptParser::Node::IDENTIFIER) {
		reduce_identifier(static_cast<GDScriptParser::IdentifierNode *>(p_binary_op->right_operand), true);
	} else {
		reduce_expression(p_binary_op->right_operand);
	}
	
	GDScriptParser::DataType left_type;
	if (p_binary_op->left_operand) {
		left_type = p_binary_op->left_operand->get_datatype();
	}
	GDScriptParser::DataType right_type;
	if (p_binary_op->right_operand) {
		right_type = p_binary_op->right_operand->get_datatype();