[heap] Do not clear slots on left trim operation

Do not clear old-to-new slots for the new FixedArray's map and length
word on left trim because these fields are tagged.

Bug: v8:9454
Change-Id: I9947a93f80efc6669498ed4c0171d728aebc782b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1767997
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63363}

src/heap/heap.cc

@@ -2998,11 +2998,6 @@ FixedArrayBase Heap::LeftTrimFixedArray(FixedArrayBase object,
   FixedArrayBase new_object =
       FixedArrayBase::cast(HeapObject::FromAddress(new_start));
 
-  // Remove recorded slots for the new map and length offset.
-  ClearRecordedSlot(new_object, new_object.RawField(0));
-  ClearRecordedSlot(new_object,
-                    new_object.RawField(FixedArrayBase::kLengthOffset));
-
   // Handle invalidated old-to-old slots.
   if (incremental_marking()->IsCompacting() &&
       MayContainRecordedSlots(new_object)) {