Typechecker does not identify errors in unreachable code

**Describe the bug**
Errors in code after a return statement are not identified.

**Standalone code, or other way to reproduce the problem**

```Hack
function unreachable_1(): int {
return 5;
return $bar;
}

function unreachable_2(): int {
$bar = 6;
return 5;
return $bar->getApple();
}
```

```
$ cat unreachable.hack
function unreachable_1(): int {
return 5;
return $bar;
}

function unreachable_2(): int {
$bar = 6;
return 5;
return $bar->getApple();
}

$ hh_client
No errors!
```

**Expected behavior**

Typechecker error about the undefined variable _or_ error about unreachable code.

**Actual behavior**

No errors.

**Environment**
- Operating system: MacOS Bug Sur
- Installation method: homebrew
- HHVM Version

```
hhvm --version
HipHop VM 4.150.0-dev (rel) (non-lowptr)
Compiler: 1645593988_N
Repo schema: 3a9d8190fa75970b0339c54801281ff070f42b81

hh_client --version
hackc-3d05e8829ec5c101725f1ce265af9f3038bc288f-4.150.0-dev
```
**Additional context**
This is related to #8660, where @Wilfred [writes](https://github.com/facebook/hhvm/issues/8660#issuecomment-589180582):

> Dead code is really tricky. Users like to have some type checking there, but it's legitimate for the type checker to ignore it entirely. Dead code checks are a best effort.

The typechecker is working here in the goal of preventing runtime errors—the dead code can't produce an error. But this is a failure in the related goal of avoiding bugs; the existence of unreachable code itself is often a bug. I found this issue because of a refactor that introduced an early return and missed some important behavior.