Improve shape compatibility checking for MultiOutputFusion

We need to be careful how we compare reduce shapes.
- If comparing against non-reduce shapes, we should compare the operand shape of a reduce with the other shape.
- If comparing two reduce shapes, we need to compare both the operand shapes and the reduce shapes.
Also, if we already have a multi-output fusion node, we should pick one of its reduce instructions for comparison, because it has more constraints than the other instructions.

PiperOrigin-RevId: 200353595