[GPU] Avoid fusing slices of already tiled ops

[Max191]

This fixes a bug in FuseAndHoistParallelLoops that can cause an infinite loop for imperfectly aligned unpack producer fusion. The FuseTilableSliceProducers pattern does not check that the producer operation is not already in the tiled loop, and the tiling implementation of imperfectly aligned unpack ops generates an extract_slice on the result of the tiled unpack. This would cause an infinite recursive application of the producer fusion pattern. This fixes the bug by enforcing that the producer operation is outside of the slice's parent loop.

[Max191]

When this unpack is tiled with a tile size of 64, an extract_slice must be generated on its result. This is what used to cause the infinite loop.

[qedawkins]

This PR has a bad smell, but the existing pattern has a worse smell :P

That said, the tiling implementation of unpack isn't great if this is the case, we probably want to think about decomposing before we get here.

[Max191]

This PR has a bad smell, but the existing pattern has a worse smell :P

Yeah, the real problem is that producer fusion is happening as a pattern rewrite in FuseAndHoistParallelLoops. In the normal TilingInterface fusion, this is done with a worklist, and the problematic extract_slice is never added to the worklist.

As a side note, I think the FuseAndHoistParallelLoops pass is due for a refactoring. There are a lot of things happening in this pass, and it can be very difficult to debug/understand for those who don't already have a lot of context about the pass. Maybe we can rethink some of these smelly patterns when we eventually refactor the pass.

That said, the tiling implementation of unpack isn't great if this is the case, we probably want to think about decomposing before we get here.

This is what I'm doing now on my data tiling branch. This bug is just something that I came across in early experimentation, and I didn't want to leave it unfixed. We only need to tile unpack ops at workgroup distribution level, and then they get decomposed before any other tiling.