Auto merge of #127036 - cjgillot:sparse-state, r=oli-obk

Make jump threading state sparse

Continuation of rust-lang/rust#127024

Both dataflow const-prop and jump threading involve cloning the state vector a lot. This PR replaces the data structure by a sparse vector, considering:
- that jump threading state is typically very sparse (at most 1 or 2 set entries);
- that dataflow const-prop is disabled by default;
- that place/value map is very eager, and prone to creating an overly large state.

The first commit is shared with the previous PR to avoid needless conflicts.

r? `@oli-obk`

Author: bors