Rollup merge of #126294 - Zalathar:spans-refiner, r=oli-obk

coverage: Replace the old span refiner with a single function

As more and more of the span refiner's functionality has been pulled out into separate early passes, it has finally reached the point where we can remove the rest of the old `SpansRefiner` code, and replace it with a single modestly-sized function.

~~There should be no change to the resulting coverage mappings, as demonstrated by the lack of changes to test output.~~

There is *almost* no change to the resulting coverage mappings. There are some minor changes to `loop` that on inspection appear to be neutral in terms of accuracy, with the old behaviour being a slightly-horrifying implementation detail of the old code, so I think they're acceptable.

Previous work in this direction includes:
- #125921
- #121019
- #119208

Author: GuillaumeGomez