Preprocess dominator tree to answer queries in O(1)

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -345,7 +345,7 @@ impl<'a, 'tcx> CoverageSpans<'a, 'tcx> {
                         // before the dominated equal spans). When later comparing two spans in
                         // order, the first will either dominate the second, or they will have no
                         // dominator relationship.
-                        self.basic_coverage_blocks.dominators().rank_partial_cmp(a.bcb, b.bcb)
+                        self.basic_coverage_blocks.rank_partial_cmp(a.bcb, b.bcb)
                     }
                 } else {
                     // Sort hi() in reverse order so shorter spans are attempted after longer spans.