[Turbofan] Don't zap old TransitionArrays to simplify concurrent compile

Formerly, we zapped a transition array when we replaced it with a
larger one, but this is no longer necessary. Leaving those arrays in
peace makes life easier for concurrent (racy) access from a background
compilation thread.

Design doc with more info about racy access to transition arrays
between the main JavaScript thread and a background compilation thread
here:
https://docs.google.com/document/d/1ax2qyENdr50Qu9yur1qNu6_zRK0m6K2l7BLM_QDBFJM/edit?usp=sharing

Change-Id: I4c2757945266d43d82ec157e0ff2b9208a8e4c63
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2253840Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68437}

src/objects/transitions.cc

@@ -406,26 +406,14 @@ Map TransitionsAccessor::GetMigrationTarget() {
   return Map();
 }
 
-void TransitionArray::Zap(Isolate* isolate) {
-  MemsetTagged(ObjectSlot(RawFieldOfElementAt(kPrototypeTransitionsIndex)),
-               ReadOnlyRoots(isolate).the_hole_value(),
-               length() - kPrototypeTransitionsIndex);
-  SetNumberOfTransitions(0);
-}
-
 void TransitionsAccessor::ReplaceTransitions(MaybeObject new_transitions) {
   if (encoding() == kFullTransitionArray) {
-    TransitionArray old_transitions = transitions();
 #if DEBUG
+    TransitionArray old_transitions = transitions();
     CheckNewTransitionsAreConsistent(
         old_transitions, new_transitions->GetHeapObjectAssumeStrong());
     DCHECK(old_transitions != new_transitions->GetHeapObjectAssumeStrong());
 #endif
-    // Transition arrays are not shared. When one is replaced, it should not
-    // keep referenced objects alive, so we zap it.
-    // When there is another reference to the array somewhere (e.g. a handle),
-    // not zapping turns from a waste of memory into a source of crashes.
-    old_transitions.Zap(isolate_);
   }
   map_.set_raw_transitions(new_transitions);
   MarkNeedsReload();

src/objects/transitions.h

@@ -338,8 +338,6 @@ class TransitionArray : public WeakFixedArray {
 
   inline void Set(int transition_number, Name key, MaybeObject target);
 
-  void Zap(Isolate* isolate);
-
   OBJECT_CONSTRUCTORS(TransitionArray, WeakFixedArray);
 };