Incorrect output in minify-dead-code-elimination

**Describe the bug**

A strange edge case code does not get eliminated correctly.

**To Reproduce**

Minimal code to reproduce the bug

```js
function b() {}

function a() {
if ('production' === 'production') {
return
}

var c = b()

if (!c) {
return
}
}
```

**Actual Output**

If there is no Error thrown,

```js
function b() {}

function a() {

var c = b();
}
```

**Expected Output**

```js
function b() {}

function a() {

}
```
**Configuration**

How are you using babel-minify?

`babel dependency`

babel version : `7.0.0-beta.3`

Used stand alone using the Node Api + babel-core

```json5
{
plugins: [],
presets: []
}
```

**Additional context**

See it in action in the [babel-repl](https://babeljs.io/repl#?babili=false&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=GYVwdgxgLglg9mABAIwBQEpEG8C-AoPUSWBRAQw2z0URmEVQHIAHAJzgBMRp4xHEAvEMQt2XHgkaYs1GolYBTKCFZhZ-WQDcyrRBEEoMBGnQYBCCNNk1Fy1erz4gA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=react%2Cstage-2&prettier=false&targets=&version=6.26.0&envVersion=)

Encountered this construction [here](https://github.com/atlassian/react-beautiful-dnd/blob/b276e4780c42ad662400631c9f315edc791a9cd8/src/view/droppable-dimension-publisher/droppable-dimension-publisher.jsx#L53-L77)

Tried to recreate it as minimal as possible using only babel-core and the dead-code plugin

Reproduce the example with babel-core and the minify-dead-code-elimination plugin, using the JavaScript snippet and configuration in the issue. Trace the plugin's handling of the unreachable branch and add or update a regression test if the repository provides one; done means the temporary variable and unreachable code are eliminated as shown in the expected output.

Written by the indexing model from the issue text.