match lowering: Split `finalize_or_candidate` into more coherent methods
I noticed that `finalize_or_candidate` was responsible for several different postprocessing tasks, making it difficult to understand.
This PR aims to clean up some of the confusion by:
- Extracting `remove_never_subcandidates` from `merge_trivial_subcandidates`
- Extracting `test_remaining_match_pairs_after_or` from `finalize_or_candidate`
- Taking what remains of `finalize_or_candidate`, and inlining it into its caller
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Reviewing individual commits and ignoring whitespace is recommended.
Most of the large-looking changes are just moving existing code around, mostly unaltered.
r? ``@Nadrieril``
d846e9252c