[turbofan] Insert sigma nodes for loop variable backedge.

src/compiler/common-operator.h

Index: src/compiler/common-operator.h
diff --git a/src/compiler/common-operator.h b/src/compiler/common-operator.h
index 81bddfdd0b10273f5fe91c2757ca00386e08ebe1..9e4d259bc9476fd18757d2c5a3a9d57b6d05d6a3 100644
--- a/src/compiler/common-operator.h
+++ b/src/compiler/common-operator.h
@@ -169,6 +169,8 @@ RegionObservability RegionObservabilityOf(Operator const*) WARN_UNUSED_RESULT;
 std::ostream& operator<<(std::ostream& os,
                          const ZoneVector<MachineType>* types);
 
+Type* TypeGuardTypeOf(Operator const*) WARN_UNUSED_RESULT;
+
 // Interface for building common operators that can be used at any level of IR,
 // including JavaScript, mid-level, and low-level.
 class CommonOperatorBuilder final : public ZoneObject {
@@ -235,6 +237,7 @@ class CommonOperatorBuilder final : public ZoneObject {
   const Operator* TailCall(const CallDescriptor* descriptor);
   const Operator* Projection(size_t index);
   const Operator* Retain();
+  const Operator* TypeGuard(Type* type);
 
   // Constructs a new merge or phi operator with the same opcode as {op}, but
   // with {size} inputs.