Auto merge of #111673 - cjgillot:dominator-preprocess, r=cjgillot,tmiasko

Preprocess and cache dominator tree

Preprocessing dominators has a very strong effect for https://github.com/rust-lang/rust/pull/111344.
That pass checks that assignments dominate their uses repeatedly. Using the unprocessed dominator tree caused a quadratic runtime (number of bbs x depth of the dominator tree).

This PR also caches the dominator tree and the pre-processed dominators in the MIR cfg cache.

Rebase of https://github.com/rust-lang/rust/pull/107157
cc `@tmiasko`

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -344,7 +344,7 @@ 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.