Auto merge of #107009 - cjgillot:jump-threading, r=pnkfelix

Implement jump threading MIR opt

This pass is an attempt to generalize `ConstGoto` and `SeparateConstSwitch` passes into a more complete jump threading pass.

This pass is rather heavy, as it performs a truncated backwards DFS on MIR starting from each `SwitchInt` terminator. This backwards DFS remains very limited, as it only walks through `Goto` terminators.

It is build to support constants and discriminants, and a propagating through a very limited set of operations.

The pass successfully manages to disentangle the `Some(x?)` use case and the DFA use case. It still needs a few tests before being ready.

Author: bors