[Bug Report] Select Select Box Multi-Select Style Problem

### Element UI version
2.12.0

### OS/Browsers version
macOS10.14.6/chrome75.0.3770.100

### Vue version
2.6.8

### Reproduction Link
https://jsfiddle.net/2meng3j8/2/

### Steps to reproduce
1. el-select 设置 multiple collapse-tags
2. 选择 el-option
3. 字段过长时会出现显示问题

### What is Expected?
正常显示，或者字段过长时显示省略号

### What is actually happening?
样式错乱

Start with the reproduction link and the reported el-select configuration using multiple and collapse-tags. Reproduce the display problem with a long el-option label in the stated Vue 2.6.8 and Chrome environment, then inspect the multi-select styling. Done means the selected tags render normally or long text is ellipsized without layout corruption.

Written by the indexing model from the issue text.