[compiler] Fix ref construction failure in ContextRef::previous

The `previous` field is immutable after initialization and the
context itself is read through an atomic load.

Bug: v8:7790
Change-Id: I8525cac7264573a7e9fc479613aaf268b72ab836
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3067333
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76054}

src/compiler/heap-refs.cc

@@ -1676,7 +1676,9 @@ ContextRef ContextRef::previous(size_t* depth) const {
     current = Context::cast(current.unchecked_previous());
     (*depth)--;
   }
-  return MakeRef(broker(), current);
+  // The `previous` field is immutable after initialization and the
+  // context itself is read through an atomic load.
+  return MakeRefAssumeMemoryFence(broker(), current);
 }
 
 base::Optional<ObjectRef> ContextRef::get(int index) const {