[turbofan] Insert sigma nodes for loop variable backedge.

src/compiler/loop-variable-optimizer.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/loop-variable-optimizer.h"
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/node-marker.h"
 #include "src/compiler/node-properties.h"
@@ skipping 351 matching lines @@
     for (auto bound : induction_var->upper_bounds()) {
       induction_var->phi()->InsertInput(
           graph()->zone(), induction_var->phi()->InputCount() - 1, bound.bound);
     }
     NodeProperties::ChangeOp(
         induction_var->phi(),
         common()->InductionVariablePhi(induction_var->phi()->InputCount() - 1));
   }
 }
 
-void LoopVariableOptimizer::ChangeFromInductionVariablePhis() {
+void LoopVariableOptimizer::ChangeToPhisAndInsertSigmas() {
   for (auto entry : induction_vars_) {
     InductionVariable* induction_var = entry.second;
     if (induction_var->phi()->opcode() == IrOpcode::kInductionVariablePhi) {
+      // Turn the induction variable phi back to normal phi.
       int value_count = 2;
       Node* control = NodeProperties::GetControlInput(induction_var->phi());
       DCHECK_EQ(value_count, control->op()->ControlInputCount());
       induction_var->phi()->TrimInputCount(value_count + 1);
       induction_var->phi()->ReplaceInput(value_count, control);
       NodeProperties::ChangeOp(
           induction_var->phi(),
           common()->Phi(MachineRepresentation::kTagged, value_count));
+
+      // If the backedge is not a subtype of the phi's type, we insert a sigma
+      // to get the typing right.
+      Node* backedge_value = induction_var->phi()->InputAt(1);
+      Type* backedge_type = NodeProperties::GetType(backedge_value);
+      Type* phi_type = NodeProperties::GetType(induction_var->phi());
+      if (!backedge_type->Is(phi_type)) {
+        Node* backedge_control =
+            NodeProperties::GetControlInput(induction_var->phi())->InputAt(1);
+        Node* rename = graph()->NewNode(common()->Sigma(phi_type),
+                                        backedge_value, backedge_control);
+        induction_var->phi()->ReplaceInput(1, rename);
+      }
     }
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8