GH-108866: Guarantee forward progress in executors.

[markshannon]

Changes the return value of executor functions, so that the next instruction can be a branch target.
Starts the optimizer at the target of the jump when optimizing from back edges to guarantee forward progress.

This simplifies ENTER_EXECUTOR a bit, as we can now rely on the executor to make forward progress.

• Issue: We need to change the contract and interface of _PyExecutorObject and _PyOptimizerObject #108866

[brandtbucher]

Hm, I'm a bit confused. Does this mean that an executor can never stop executing in a frame other than the one it started in? That seems like a big step backward if so, since it means that we can't have specialized instructions in inlined calls...

(If this isn't intended, maybe you're just missing a frame = tstate->current_frame and locals reload?)

[markshannon]

Just missing frame = tstate->current_frame. And a test for it. It is basically impossible to test for, as that path is only hit for custom optimizers.

[markshannon]

Reviewed offline.