Speed up is_a_spec

Speed up is_a_spec by about 6 seconds on my work laptop.

```
farleyknight:~/dev/crystal/crystal$ rspec spec/codegen/is_a_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.....................

Finished in 4.01 seconds
21 examples, 0 failures
farleyknight:~/dev/crystal/crystal$ rspec spec/codegen/is_a_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.....................

Finished in 10.62 seconds
21 examples, 0 failures
```