Introduce 'kLengthString' in torque code

This CL replaces occurrences of "length" with the CSA macro
LengthStringConstant().

R=jgruber@chromium.org

Bug: v8:8015
Change-Id: Idf095587940f859e4c634865560abae325cd9fb4
Reviewed-on: https://chromium-review.googlesource.com/1201782Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#55578}

src/builtins/array.tq

@@ -20,7 +20,7 @@ module array {
       return a.length_fast;
     } else
       deferred {
-        return ToLength_Inline(context, GetProperty(context, o, 'length'));
+        return ToLength_Inline(context, GetProperty(context, o, kLengthString));
       }
   }
 

src/builtins/base.tq

@@ -149,12 +149,14 @@ extern macro TrueConstant(): Boolean;
 extern macro FalseConstant(): Boolean;
 extern macro Int32TrueConstant(): bool;
 extern macro Int32FalseConstant(): bool;
+extern macro LengthStringConstant(): String;
 
 const Hole: Oddball = TheHoleConstant();
 const Null: Oddball = NullConstant();
 const Undefined: Oddball = UndefinedConstant();
 const True: Boolean = TrueConstant();
 const False: Boolean = FalseConstant();
+const kLengthString: String = LengthStringConstant();
 
 const true: constexpr bool generates 'true';
 const false: constexpr bool generates 'false';

third_party/v8/builtins/array-sort.tq

@@ -554,7 +554,7 @@ module array {
     if (IsJSArray(receiver)) return unsafe_cast<JSArray>(receiver).length_fast;
 
     const len: Number =
-        ToLength_Inline(context, GetProperty(context, receiver, 'length'));
+        ToLength_Inline(context, GetProperty(context, receiver, kLengthString));
     return unsafe_cast<Smi>(len);
   }
 
@@ -1782,8 +1782,7 @@ module array {
     }
     label slow {
       // 3. Let len be ? ToLength(? Get(obj, "length")).
-      const len: Number =
-          ToLength_Inline(context, GetProperty(context, obj, 'length'));
+      const len: Number = GetLengthProperty(context, obj);
 
       if (len < 2) return receiver;
       const nofNonUndefined: Smi = PrepareElementsForSort(context, obj, len);