Auto merge of rust-lang#65874 - Nadrieril:clarify-usefulness, r=varkor

Clarify pattern-matching usefulness algorithm

This PR clarifies a bit the usefulness algorithm by emphasizing that each row of the matrix can be seen as a sort of stack from which we pop constructors. It also moves code around to increase separation of concerns.

This is part of my splitting of rust-lang#65160 into smaller PRs.

Author: bors