[wasm] Break a cycle with a loop node in the int64-lowering.

The control edges in a TurboFan graph can form a cycle. To break this cycle in the int64-lowering we add special handling for loop nodes. Similar handling already exists for phi nodes and effectphi nodes, which breaks cycles formed by value edges and effect edges, respectively.

Review-Url: https://codereview.chromium.org/2511503002
Cr-Commit-Position: refs/heads/master@{#41071}

src/compiler/int64-lowering.cc

@@ -61,7 +61,8 @@ void Int64Lowering::LowerGraph() {
           // that they are processed after all other nodes.
           PreparePhiReplacement(input);
           stack_.push_front({input, 0});
-        } else if (input->opcode() == IrOpcode::kEffectPhi) {
+        } else if (input->opcode() == IrOpcode::kEffectPhi ||
+                   input->opcode() == IrOpcode::kLoop) {
           stack_.push_front({input, 0});
         } else {
           stack_.push_back({input, 0});

test/unittests/compiler/int64-lowering-unittest.cc

@@ -875,6 +875,25 @@ TEST_F(Int64LoweringTest, EffectPhiLoop) {
 
   LowerGraph(load, MachineRepresentation::kWord64);
 }
+
+TEST_F(Int64LoweringTest, LoopCycle) {
+  // New node with two placeholders.
+  Node* compare = graph()->NewNode(machine()->Word64Equal(), Int64Constant(0),
+                                   Int64Constant(value(0)));
+
+  Node* load = graph()->NewNode(
+      machine()->Load(MachineType::Int64()), Int64Constant(value(1)),
+      Int64Constant(value(2)), graph()->start(),
+      graph()->NewNode(
+          common()->Loop(2), graph()->start(),
+          graph()->NewNode(common()->IfFalse(),
+                           graph()->NewNode(common()->Branch(), compare,
+                                            graph()->start()))));
+
+  NodeProperties::ReplaceValueInput(compare, load, 0);
+
+  LowerGraph(load, MachineRepresentation::kWord64);
+}
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8