Fixes #3547.

This code:

class Foo
  def bar
    print as Foo
  end
end
Foo.new.bar

will now print the instance of Foo, instead of raising an unexpected token error.

I'm not sure what the implications of this is on a higher level, as this is my first PR against Crystal. It seems like this check is perhaps legacy -- by fact of this conditional check singling out as, it makes a clear break from the behavior of as?. It also looks like this check has been in here quite a while. Let me know what you think.

cc @asterite