Fix syntax_editor duplicated changed element

[A4-Tacks]

Fixes #21020

Example

let arg_list = make::arg_list([make::expr_literal("1").into(), make::expr_literal("2").into()]);
let mut editor = SyntaxEditor::new(arg_list.syntax().clone());
let target_expr = make::expr_literal("3").clone_for_update();

for arg in arg_list.args() {
    editor.replace(arg.syntax(), target_expr.syntax());
}

let edit = editor.finish();
let expect = expect![["(3, 3)"]];
expect.assert_eq(&edit.new_root.to_string());

Before this PR

(, )3

After this PR

(3, 3)