Auto merge of #122046 - Nadrieril:integrate-or-pats2, r=matthewjasper

match lowering: handle or-patterns one layer at a time

`create_or_subcandidates` and `merge_trivial_subcandidates` both call themselves recursively to handle nested or-patterns, which is hard to follow. In this PR I avoid the need for that; we now process a single "layer" of or-patterns at a time.

By calling back into `match_candidates`, we only need to expand one layer at a time. Conversely, since we always try to simplify a layer that we just expanded (thanks to rust-lang/rust#123067), we only have to merge one layer at a time.

r? `@matthewjasper`

Author: bors