Handle self-loops too.

Author: cjgillot

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -842,16 +842,18 @@ fn maybe_loop_headers(body: &Body<'_>) -> DenseBitSet<BasicBlock> {
     let mut maybe_loop_headers = DenseBitSet::new_empty(body.basic_blocks.len());
     let mut visited = DenseBitSet::new_empty(body.basic_blocks.len());
     for (bb, bbdata) in traversal::postorder(body) {
-        let _new = visited.insert(bb);
-        debug_assert!(_new);
-
         // Post-order means we visit successors before the block for acyclic CFGs.
         // If the successor is not visited yet, consider it a loop header.
         for succ in bbdata.terminator().successors() {
             if !visited.contains(succ) {
                 maybe_loop_headers.insert(succ);
             }
         }
+
+        // Only mark `bb` as visited after we checked the successors, in case we have a self-loop.
+        //     bb1: goto -> bb1;
+        let _new = visited.insert(bb);
+        debug_assert!(_new);
     }
 
     maybe_loop_headers