Rollup merge of #121374 - Nadrieril:factor-explain, r=matthewjasper

match lowering: Split off `test_candidates` into several functions and improve comments

The logic of `test_candidates` has three steps: pick a test, sort the candidates, and generate code for everything. So I split it off into three methods.

I also ended up reworking the comments that explain the algorithm. In particular I added detailed examples. I removed the digression about rust-lang/rust#29740 because it's no longer relevant to how the code is structured today.

r? ``@matthewjasper``