Auto merge of #127024 - cjgillot:jump-prof, r=oli-obk

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 rust-lang/rust#116721

r? `@oli-obk`

Author: bors