[Relay] Restore dominator check

I goofed in apache#11481 in moving a dominates check to
an ICHECK (and I'm confused how it got through ci?).

It is ok to match a sub-graph which has dataflow
outside of the sub-graph, provided all such flows
eventually come into the sub-graph.

src/relay/ir/dataflow_matcher.cc

@@ -609,6 +609,8 @@ void PatternGrouper::VisitExprs() {
 }
 
 void PatternGrouper::CreateGroup(const Expr& expr) {
+  VLOG(1) << "Creating group for:" << std::endl << PrettyPrint(expr);
+
   int var_number = 0;
 
   auto node_map = matcher_->GetMemo();
@@ -696,6 +698,7 @@ void PatternGrouper::CreateGroup(const Expr& expr) {
   auto body = extractor.Mutate(expr);
 
   group.function = Function(params, body, NullValue<Type>(), Array<TypeVar>());
+  VLOG(1) << "Candidate extracted function:" << std::endl << PrettyPrint(group.function);
   group.name = extractor.GetName();
   // Check to make sure we aren't overlapping with another group or creating an invalid fusion
   // The MatchExtractor will create a new graph by replacing nodes that match the inputs of the
@@ -708,6 +711,10 @@ void PatternGrouper::CreateGroup(const Expr& expr) {
   // Similiarly, if interior nodes in a group are used outside of the group fusing to a single
   // output would create an invalid graph tranformation, so we block the creation of such groups.
   auto memo = extractor.GetMemo();
+  for (auto kv : memo) {
+    VLOG(1) << "matched index " << matcher_->expr_to_node(kv.first)->index_;
+  }
+
   for (auto kv : memo) {
     // Check to ensure that this node isn't an input or a global
     if (inputs.count(kv.first) == 0 && kv.first.as<OpNode>() == nullptr &&
@@ -720,16 +727,19 @@ void PatternGrouper::CreateGroup(const Expr& expr) {
         // if the node isn't the output of the group
         auto node = matcher_->expr_to_node(kv.first);
         for (auto* output : node->outputs_) {
-          // and the node is used by nodes outside of the group
           if (memo.count(output->ref()) == 0) {
-            // TODO(mbs): This condition used to also include the following test, which since
-            // the dominators relation is used back-to-front was always vacuously true. So the
-            // code is just rejecting the match if a strictly internal node happened to connect
-            // to an outside node.
-            ICHECK(!matcher_->expr_to_node(expr)->Dominates(output));
-            // Exit because nodes in this pattern's body are used outside the pattern, fusing it
-            // would be invalid
-            return;
+            // A node inside the matched group contributes an output to nodes outside of the matched
+            // group...
+            auto root = matcher_->expr_to_node(expr);
+            if (!root->Dominates(output)) {
+              // ...and the outside dataflow does not come back to the root of the matched group.
+              // So reject the match since it would create a cycle.
+              VLOG(1) << "Rejecting group since would create a cycle with output " << output->index_
+                      << " for root " << root->index_ << " in graph:" << std::endl
+                      << matcher_->expr_graph().ToString();
+              return;
+            }
+            // else: We'll allow the output to be included in the matched group.
           }
         }
       }

src/relay/ir/dataflow_matcher_impl.h

@@ -55,6 +55,7 @@ class DFPatternMatcher : public DFPatternFunctor<bool(const DFPattern&, const Ex
   const std::unordered_map<DFPattern, Array<Expr>, ObjectPtrHash, ObjectPtrEqual>& memo() const {
     return memo_;
   }
+  const IndexedGraph<Expr>& expr_graph() const { return *expr_graph_; }
 
  protected:
   bool VisitDFPattern(const DFPattern& pattern, const Expr& expr) override;