Avoid cloning jump threading state when possible The current implementation of jump threading passes most of its time cloning its state. This PR attempts to avoid such clones by special-casing the last predecessor when recursing through a terminator. This is not optimal, but a first step while I refactor the state data structure to be sparse. The two other commits are drive-by. Fixes https://github.com/rust-lang/rust/issues/116721 r? `@oli-obk`