Make jump threading state sparse Continuation of https://github.com/rust-lang/rust/pull/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`