Argument in chained promise not checked after truthiness check

In a promise chain, the value passed to `then` is not type checked if its truthiness is checked.

Given a function that returns a promise:

`/** @return {!Promise} */ var foo;`

Closure doesn't detect this undefined property usage on that string in a chained `then()` function if we do a truthiness check on the string:

```
Promise.resolve().then(function() {
return foo();
}).then(function(word) {
if (word) // this if statement makes the below line pass
console.log(word.arglebarg); // this should fail, but doesn't
});
```

This only repros when the Promise is returned from another Promise.

Full code example:

```
// compile with:
// java -jar compiler.jar --jscomp_error=missingProperties --checks_only --language_in=ECMASCRIPT6_STRICT --language_out=ECMASCRIPT5_STRICT

/** @return {!Promise} */
function foo() {
return new Promise((resolve, reject) => {
resolve('hello');
});
}

function main() {
Promise.resolve().then(function() {
return foo();
}).then(function(word) {
if (word)
console.log(word.arglebarg);
});
}
```

Version: v20190106
Built on: 2019-01-10 21:49