When the label width attribute of the [bug report] form component is auto, the input is wrapped under ie 11

### Element UI version
2.15.6

### OS/Browsers version
windows10 / IE 11

### Vue version
2.6.14

### Reproduction Link
https://jsfiddle.net/ey26a3uh/

### Steps to reproduce
使用IE11打开以上代码的页面即可看到问题，注：经测试jsfiddle、codepen都不支持IE11，所以可能需要复制链接里的代码在本地运行

### What is Expected?
IE 11下input不换行

### What is actually happening?
IE 11下input被换行