[LoopFusion] Cost model development

We are working on developing a cost model for loop fusion. The goal is to start with a very simple and conservative cost model. At a high level we need to:

1. Have a code size limit to make sure aggressive fusion doesn't result in high register pressure.
2. Check for a number of reused values to ensure fusion has some benefit.

I am currently looking into one of our benchmarks to see whether we can catch the opportunity there and then develop the cost model for it.

@sjoerdmeijer @Meinersbur @CongzheUalberta @1997alireza @Alf0nso