Repro steps:
1. Create a new VFX
2. Select two blocks in the Output context, right-click, and turn them into a Subgraph block.
3. Select the new subgraph block and copy it
4. Right click on the subgraph and select 'Enter Subgraph'
5. Select one of the subgraph blocks and paste the Subgraph from step 3.
6. Unity hands and eventually crashes.

Expected outcome:
No crash / not allow pasting a block subgraph within itself if it's not supported

Notes:
- The same workflow seems fine for operator subgraphs