Add an initial check to simplify pattern matching instanceofs.

[copybara-service]

Add an initial check to simplify pattern matching instanceofs.

This only handles the simplest case of,

if (x instanceof Foo) {
  var foo = (Foo) x;
  ...
}

While we should also be catching:

if (!(x instanceof Foo)) {
  ...
}
...
var foo = (Foo) x;

That's certainly a bit harder to do, though!