[turbofan] Introduce a TransitionElementsKind simplified operator.

src/compiler/simplified-operator.h

Index: src/compiler/simplified-operator.h
diff --git a/src/compiler/simplified-operator.h b/src/compiler/simplified-operator.h
index 53a7007760d7f14dd0ed477fb583c00373cf50dc..9cd721f22924847424811bb90ac6627648fdb470 100644
--- a/src/compiler/simplified-operator.h
+++ b/src/compiler/simplified-operator.h
@@ -140,7 +140,15 @@ std::ostream& operator<<(std::ostream&, CheckForMinusZeroMode);
 
 CheckForMinusZeroMode CheckMinusZeroModeOf(const Operator*) WARN_UNUSED_RESULT;
 
-Type* TypeOf(const Operator* op) WARN_UNUSED_RESULT;
+// A descriptor for elements kind transitions.
+enum class ElementsTransition : uint8_t {
+  kFastTransition,  // simple transition, just updating the map.
+  kSlowTransition   // full transition, round-trip to the runtime.
+};
+
+std::ostream& operator<<(std::ostream&, ElementsTransition);
+
+ElementsTransition ElementsTransitionOf(const Operator* op) WARN_UNUSED_RESULT;
 
 BinaryOperationHints::Hint BinaryOperationHintOf(const Operator* op);
 
@@ -288,6 +296,9 @@ class SimplifiedOperatorBuilder final : public ZoneObject {
   const Operator* ObjectIsString();
   const Operator* ObjectIsUndetectable();
 
+  // transition-elements-kind object, from-map, to-map
+  const Operator* TransitionElementsKind(ElementsTransition transition);
+
   const Operator* Allocate(PretenureFlag pretenure = NOT_TENURED);
 
   const Operator* LoadField(FieldAccess const&);