Checks against an opaque type's subtype type fail to properly filter that type out

Can't reproduce on try flow because this requires an opaque type and thus a different file.

```js
/* file1.js */
export opaque type foo: string = string;

/* file2.js */

import type { foo } from 'file1';
function bar(param: foo | number) {
if (typeof param === 'string') {
param = 42;
}
// param should now be a number no matter what

// with an opaque type this creates an error because `param` is still (incorrectly)
// `foo | number`, however there is no programatic way to check against `foo` in
// this case. Since all foo's are strings the above `if` should work.
param.toFixed(2);
}
```

flow 0.64.0