[runtime] Enable turning ConsStrings into ThinStrings

We previously missed out on turning ConsString objects into ThinStrings
due to the flattening step in StringTable::LookupString.

  // Example input string:
  string = cons("A", cons"B", "C"))
  // After flatten:
  string == cons("ABC", "")
  // OLD: After internalising and thinning:
  string == cons(#"ABC", "")

The new behaviour yields a thin string after internalising:

  // NEW: After internalising and thinning:
  string == thin(#"ABC")

Change-Id: I99bbebd52fa02ecfeb2289cb85b69f3e01175cdd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3667080
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81004}

src/objects/string-table.cc

@@ -484,27 +484,24 @@ Handle<String> StringTable::LookupString(Isolate* isolate,
   //
   // For lookup hits, we use the StringForwardingTable for shared strings to
   // delay the transition into a ThinString to the next stop-the-world GC.
-
-  string = String::Flatten(isolate, string);
-  if (string->IsInternalizedString()) return string;
-
-  string->EnsureHash();
-  uint32_t raw_hash_field = string->raw_hash_field(kAcquireLoad);
+  Handle<String> result = String::Flatten(isolate, string);
+  if (!result->IsInternalizedString()) {
+    result->EnsureHash();
+    uint32_t raw_hash_field = result->raw_hash_field(kAcquireLoad);
 
     if (String::IsForwardingIndex(raw_hash_field)) {
       const int index = String::HashBits::decode(raw_hash_field);
-    return handle(
+      result = handle(
           isolate->string_forwarding_table()->GetForwardString(isolate, index),
           isolate);
+    } else {
+      InternalizedStringKey key(result, raw_hash_field);
+      result = LookupKey(isolate, &key);
     }
-
-  InternalizedStringKey key(string, raw_hash_field);
-  Handle<String> result = LookupKey(isolate, &key);
-
-  if (!string->IsInternalizedString()) {
+  }
+  if (*string != *result && !string->IsThinString()) {
     SetInternalizedReference(isolate, *string, *result);
   }
-
   return result;
 }