[turbofan] Insert sigma nodes for loop variable backedge.

src/compiler/operation-typer.cc

Index: src/compiler/operation-typer.cc
diff --git a/src/compiler/operation-typer.cc b/src/compiler/operation-typer.cc
index c189d5c854eb69beeb7088c67dc6bdad43736d91..bec6f30b208bec1d4ffb52eefe06a7bc8ef0db39 100644
--- a/src/compiler/operation-typer.cc
+++ b/src/compiler/operation-typer.cc
@@ -4,6 +4,7 @@
 
 #include "src/compiler/operation-typer.h"
 
+#include "src/compiler/common-operator.h"
 #include "src/factory.h"
 #include "src/isolate.h"
 #include "src/type-cache.h"
@@ -456,6 +457,10 @@ Type* OperationTyper::FalsifyUndefined(ComparisonOutcome outcome) {
   return singleton_true();
 }
 
+Type* OperationTyper::TypeSigma(const Operator* sigma_op, Type* input) {
+  return Type::Intersect(input, SigmaTypeOf(sigma_op), zone());
+}
+
 Type* OperationTyper::TypeJSAdd(Type* lhs, Type* rhs) {
   lhs = ToPrimitive(lhs);
   rhs = ToPrimitive(rhs);