Short circuit trivial refutability checks

Frans Skarmanrequested to merge
push-vrsvzrlzykvx into main

Refutability checking often gets in the way when a pattern is clearly irrefutable. This does a much cheaper refutability check for clearly irrefutable pattersn and only falls back on real refutability checking if necessary

Author checklist

  • New Diagnostics have at least one snapshot test that triggers it
  • Added a changelog entry, if relevant
    • You can use ./add_changelog <MR number or a_descriptive_name> to do this

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