[Bug Report] 多选树形结构的表格组件，子行的select选中和取消选中都没有效果

### Element UI version
2.15.7

### OS/Browsers version
谷歌浏览器版本 90.0.4430.93

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/Alun/pen/gOXZBEb

### Steps to reproduce
步骤一：点击序号为4或序号为5左边的勾选框，发现无法选中

步骤二：

1.点击全选按钮

2.再点击序号4(表格第四子行)或序号5(表格第五子行)左边的勾选框，发现无法取消选中

### What is Expected?
1.全选按钮如果是未勾选的状态，点击子行select可以选中

2.全选按钮如果是已勾选的状态，点击子行select可以取消选中

### What is actually happening?
1.全选按钮如果是未勾选的状态，点击子行select不可以选中

2.全选按钮如果是已勾选的状态，点击子行select不可以取消选中

Start with the CodePen reproduction at https://codepen.io/Alun/pen/gOXZBEb using Element UI 2.15.7, Vue 2.6.10, and Chrome 90. Reproduce selection and deselection on child rows 4 and 5, then trace the table component's tree-selection handling. Done means child-row checkboxes toggle correctly both before and after using the select-all control.

Written by the indexing model from the issue text.