[turbofan] Set proper representation for initial arguments length.

The JSArgumentsObject::length representation is initially Smi, so we can
record that on the initial map and use it to optimize the accesses in
TurboFan based on that. Similar for JSSloppyArgumentsObject::caller.

BUG=v8:6262
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2810333004
Cr-Commit-Position: refs/heads/master@{#44644}

src/bootstrapper.cc

@@ -3029,13 +3029,13 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     {  // length
       Descriptor d = Descriptor::DataField(
           factory->length_string(), JSSloppyArgumentsObject::kLengthIndex,
-          DONT_ENUM, Representation::Tagged());
+          DONT_ENUM, Representation::Smi());
       map->AppendDescriptor(&d);
     }
     {  // callee
       Descriptor d = Descriptor::DataField(
           factory->callee_string(), JSSloppyArgumentsObject::kCalleeIndex,
-          DONT_ENUM, Representation::Tagged());
+          DONT_ENUM, Representation::HeapObject());
       map->AppendDescriptor(&d);
     }
     // @@iterator method is added later.
@@ -3086,7 +3086,7 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     {  // length
       Descriptor d = Descriptor::DataField(
           factory->length_string(), JSStrictArgumentsObject::kLengthIndex,
-          DONT_ENUM, Representation::Tagged());
+          DONT_ENUM, Representation::Smi());
       map->AppendDescriptor(&d);
     }
     {  // callee

src/compiler/access-builder.cc

@@ -716,8 +716,8 @@ FieldAccess AccessBuilder::ForValue() {
 FieldAccess AccessBuilder::ForArgumentsLength() {
   FieldAccess access = {kTaggedBase,         JSArgumentsObject::kLengthOffset,
                         Handle<Name>(),      MaybeHandle<Map>(),
-                        Type::NonInternal(), MachineType::AnyTagged(),
-                        kFullWriteBarrier};
+                        Type::SignedSmall(), MachineType::TaggedSigned(),
+                        kNoWriteBarrier};
   return access;
 }