When `discharge?` failed, the `usedSimps` was being restored, but the
cache wasn't. This bug was exposed by issue #3710.
This PR makes the following changes:
- We restore the `cache` at `discharge?`. We use `SMap` to ensure the
operation is efficient.
- We don't need the field `dischargeDepth` anymore at `Simp.Result`.
- `UsedSimps` should use `PHashMap` since it is not used linearly.
closes#3710
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Co-authored-by: Mario Carneiro <di.gama@gmail.com>
