[Bug Report] [饿了么表单组件的自定义列和合并行/列冲突] El-Table Components , Table Cell rowspan and colspan has bad show-view when column use slot-scope

### Element UI version
2.13.0

### OS/Browsers version
win 10 / chrome

### Vue version
3.0.0-alpha.4

### Reproduction Link
https://jsfiddle.net/mmx38qxw/

### Steps to reproduce
在element-ui 组件 table 处 任意一个 有自定义列的地方 使用 colspan 或者 rowspan 会导致错列 ， 列被挤开。导致显示位置与设定的列对应不上

[饿了么表单组件的自定义列和合并行/列冲突] El-Table Components , Table Cell rowspan and colspan has bad show-view when column use slot-scope

### What is Expected?
fix this problem

### What is actually happening?
bug

Start with the JSFiddle reproduction at https://jsfiddle.net/mmx38qxw/ and inspect the El-Table component entry point for custom columns combined with rowspan or colspan. Confirm the column displacement described in the reproduction, then verify that merged cells and slot-scope columns remain aligned after the fix.

Written by the indexing model from the issue text.