[Bug Report] el-table span-method方法中写自增会出现无限循环

### Element UI version
2.13.2

### OS/Browsers version
win10 chomd版本 84.0.4147.105

### Vue version
2.6.1

### Reproduction Link
https://codepen.io/zym_java/pen/GRqxzJq

### Steps to reproduce
arraySpanMethod({ row, column, rowIndex, columnIndex }){//当前行row、当前列column、当前行号rowIndex、当前列号columnIndex
if(columnIndex==1){
console.log(rowIndex, columnIndex)
this.indexCount = this.indexCount this.indexCount1-1
}
}

### What is Expected?
合并相关的行

### What is actually happening?
出现无限循环