[compiler] Fix FeedbackVectorRef::shared_function_info

.. another MakeRef vs. MakeRefAssumeMemoryFence spot.

Bug: v8:7790,chromium:1246465
Change-Id: I587538f5756896036aad5db4939a462c01d4cc2f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3141580
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76668}

src/compiler/heap-refs.cc

@@ -2665,7 +2665,8 @@ base::Optional<SharedFunctionInfoRef> FeedbackCellRef::shared_function_info()
 }
 
 SharedFunctionInfoRef FeedbackVectorRef::shared_function_info() const {
-  return MakeRef(broker(), object()->shared_function_info());
+  // Immutable after initialization.
+  return MakeRefAssumeMemoryFence(broker(), object()->shared_function_info());
 }
 
 bool NameRef::IsUniqueName() const {