`SubgraphFusion` crashes with `IndexError: list index out of range` error on certain graphs.

**Describe the bug**

This came up as I was working with #1673 (i.e. fixing some sub-optimal behaviour of `MapFusion` causes an unrelated test to break through `SubgraphFusion`.

While I'd like to fix it myself already, I cannot tell from the subgraph fusion code what it was _trying_ to do here. So, here is an issue for longer-term tracking. In general, the subgraph fusion code looks quite untested and unmaintained. If that's really the case, is it worth to keep `SubgraphFusion` in the default optimization routine?

**To Reproduce**

Use this SDFG: https://polybox.ethz.ch/index.php/s/pMlKv8jkJ3B1SdR/download?path=%2F&files=nbody-subgraphfusion-bug.sdfg

With the this code:
```python
import dace
from dace import SDFG
from dace.transformation.auto.auto_optimize import auto_optimize

g = SDFG.from_file('nbody-subgraphfusion-bug.sdfg')
g.validate()
auto_optimize(g, device=dace.dtypes.DeviceType.CPU, validate_all=True)
```

**Expected behavior**

It wouldn't crash, and be successful with the fusion.