Decouple tiling from fusion in Linalg.

This CL modifies the linalg-fusion pass such that it does not tile anymore as part of the pass. Tiling is a separate concern that enables linalg fusion but should happen before.
This makes fusion more composable with other decisions.
In particular the fusion pass now becomes greedy and only applies the transformation on a best-effort basis.

This should also let fusion work in a multi-hop fashion with chains of producer/consumers.

Since the fusion pass does not perform tiling anymore, tests are rewritten to be in pretiled form and make the intent of the test clearer (albeit more verbose).

PiperOrigin-RevId: 271357741