[turbofan] Insert sigma nodes for loop variable backedge.

src/compiler/operation-typer.h

Index: src/compiler/operation-typer.h
diff --git a/src/compiler/operation-typer.h b/src/compiler/operation-typer.h
index b37360a8c4e987d891affe32dae11e3332baab62..52fa7cf80b32667149cdd47a3de35e8adccbcf6c 100644
--- a/src/compiler/operation-typer.h
+++ b/src/compiler/operation-typer.h
@@ -19,6 +19,8 @@ class Zone;
 
 namespace compiler {
 
+class Operator;
+
 class OperationTyper {
  public:
   OperationTyper(Isolate* isolate, Zone* zone);
@@ -40,6 +42,8 @@ class OperationTyper {
 
   Type* NumberAbs(Type* type);
 
+  Type* TypeSigma(const Operator* sigma_op, Type* input);
+
   enum ComparisonOutcomeFlags {
     kComparisonTrue = 1,
     kComparisonFalse = 2,