[turbofan] Insert sigma nodes for loop variable backedge.

src/compiler/common-operator.cc

Index: src/compiler/common-operator.cc
diff --git a/src/compiler/common-operator.cc b/src/compiler/common-operator.cc
index bc36c5208bd2b5ad2ab98a0b41896f8aa5d09911..5862e6719cd17c7c665856692f41f984e7184ff9 100644
--- a/src/compiler/common-operator.cc
+++ b/src/compiler/common-operator.cc
@@ -221,6 +221,11 @@ RegionObservability RegionObservabilityOf(Operator const* op) {
   return OpParameter<RegionObservability>(op);
 }
 
+Type* SigmaTypeOf(Operator const* op) {
+  DCHECK_EQ(IrOpcode::kSigma, op->opcode());
+  return OpParameter<Type*>(op);
+}
+
 std::ostream& operator<<(std::ostream& os,
                          const ZoneVector<MachineType>* types) {
   // Print all the MachineTypes, separated by commas.
@@ -853,6 +858,13 @@ const Operator* CommonOperatorBuilder::Phi(MachineRepresentation rep,
       rep);                                              // parameter
 }
 
+const Operator* CommonOperatorBuilder::Sigma(Type* type) {
+  return new (zone()) Operator1<Type*>(   // --
+      IrOpcode::kSigma, Operator::kPure,  // opcode
+      "Sigma",                            // name
+      1, 0, 1, 1, 0, 0,                   // counts
+      type);                              // parameter
+}
 
 const Operator* CommonOperatorBuilder::EffectPhi(int effect_input_count) {
   DCHECK(effect_input_count > 0);  // Disallow empty effect phis.