[Bug Report] el-input prefix/suffix is displayed at wrong position when used with append/prepend slot.

### Element UI version
2.13.0

### OS/Browsers version
macOS Catalina / Google Chrome 81

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/POPOPON/pen/bGVYgpG?editors=1010

### Steps to reproduce
Use el-input prefix and append slots simultaneously.

### What is Expected?
prefix/suffix are always centered.

### What is actually happening?
prefix/suffix is displayed at the top of the input when used with append/prepend

Start with the provided CodePen reproduction and inspect the el-input implementation for prefix/suffix elements when append or prepend slots are also present. Done means the prefix and suffix remain vertically centered in the input in that combined-slot case, while the existing input behavior continues to work.

Written by the indexing model from the issue text.