Issue title: Miss handle of `background-size` under IE8
 Issue body: As we known that `background-size` property is not supported under IE8 mentioned inside [MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/background-size#Browser_compatibility), I hope that autoprefixer can handle it like `opacity`:

```css
/* before */

.figure {
    opacity: .5;
}

/* after */

.figure {
    filter: alpha(opacity=50);
}
```

When it comes to `background-size`:

```css
/* before */

.figure {
    background-size: cover;
    background-image: url(./path/image.jpg);
}

/* after */

.figure {
    background-size: cover;
    background-image: url(./path/image.jpg);
    filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='[hash].jpg', sizingMethod=scale);
}
```

But there is a special situation you need to know is that `src` should be relative to referenced page site, rather than local file system. If we all use postcss to bundle image source and rename with a specified hash, this notice can be ignored.
 Comments: 
 Comment 0: > that src should be relative to referenced page site, rather than local file system.

That sounds like something Autoprefixer won't be able to do.

Autoprefixer has no access to the web page url and isn't built in a way to be able to figure that sort of thing out from the file system.
 Comment 1: Actually, you can just convert when src is only specified with file name, where developers can use some tutorials to import images with file name only like [webpack-contrib/file-loader](https://github.com/webpack-contrib/file-loader).
 Comment 2: Sorry, Autoprefixer is only about prefixes.

You need to checkout PostCSS plugins. It was a plugin o polyfill `alpha(opacity=50)`.
 Comment 3: You mean I should open this issue for the plugin [oldIE](https://github.com/jonathantneal/oldie)? @ai 
 Comment 4: Yeap
 Comment 5: I have thought about this before, but it is wierd that I just use **ejohnson@example.com** + **ejohnson@example.com** without **oldie** plugin, and `opacity` has been shimed with `filter`. So I think perhaps the plugin is embeded within autoprefixer?
 Comment 6: Agree. It was a mistake and it was not possible to fix it 😅

https://github.com/postcss/autoprefixer/blob/11c61e3d60a688e14128bd7927be29110fcc33df/test/cases/filter.css
 Comment 7: 😢 So if I upgrade autoprefixer, should I use the **oldie** plugin to do so?
 Comment 8: > should I use the oldie plugin to do so?

Yeap. Any new IE compatibility features should go to oldie.
{'source': 'GithubIssues', 'repo_name_with_owner': 'postcss/autoprefixer', 'issue_id': 672698218, 'repo_stars': 19007.0}