Auto merge of #121172 - Nadrieril:simplify-empty-selection, r=matthewjasper

match lowering: simplify empty candidate selection

In match lowering, `match_simplified_candidates` is tasked with removing candidates that are fully matched and linking them up properly. The code that does that was needlessly complicated; this PR simplifies it.

The overall change isn't big but I split it up into tiny commits to convince myself that I was correctly preserving behavior. The test changes are all due to the first commit. Let me know if you'd prefer me to split up the PR to make reviewing easier.

r? `@matthewjasper`

Author: bors