broken minified code: v + "" == "x";

**Describe the bug**
There is a bug in the minification of the following code.

**To Reproduce**
[babel try it out](https://babeljs.io/repl/#?babili=true&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=GYVwdgxgLglg9mABBOBbADgQwE4FMAUAbgJSIDeAUIonlCNkoYgNSIBEbiAvF-wB5sA3BQC-QA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&sourceType=module&lineWrap=true&presets=es2017%2Creact%2Cbabili&prettier=true&targets=&version=6.26.0&envVersion=)

**input**
```js
function compare(v) {
return v + "" == "x";
}
compare(4) // false
compare('a') // false
```

**Actual Output**

```js
function compare(a) {
return a + "x";
}
compare(4) // '4x'
compare('a') // 'ax'
```

**Expected Output**
don't minify this case

Start by reproducing the issue in the linked Babel REPL with the provided compare function, then trace the minification transform that rewrites the string comparison. Done means the minifier preserves the comparison's behavior for the shown numeric and string inputs and a regression test covers this case.

Written by the indexing model from the issue text.