make @non_differentiable use identical pullbacks when possible
#1974
| Job | Run time |
|---|---|
| 3m 8s | |
| 2m 3s | |
| 2m 23s | |
| 2m 26s | |
| 2m 15s | |
| 1m 53s | |
| 2m 14s | |
| 1m 47s | |
| 12m 18s | |
| 30m 27s |