Unreachable subpatterns are rare

We may as well leave early when we know there's nothing to report.

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -389,6 +389,8 @@ fn check_arms<'p, 'tcx>(
                     hir::MatchSource::AwaitDesugar | hir::MatchSource::TryDesugar => {}
                 }
             }
+            Useful(unreachables) if unreachables.is_empty() => {}
+            // The arm is reachable, but contains unreachable subpatterns (from or-patterns).
             Useful(unreachables) => {
                 let mut unreachables: Vec<_> = unreachables.into_iter().flatten().collect();
                 // Emit lints in the order in which they occur in the file.