[Compile] Replace resetting of feedback vector with CHECK_EQ.

Previous differences in eager and lazy parsing meant that it was possible
that feedback vectors could be different for the same function depending on
how it was compiled. These issues have now been fixed, so remove the workaround
and add an explicit CHECK that the feedback vectors are the same.

BUG=chromium:984344,v8:9511

Change-Id: I0edfa350b1a2f236db1ee2f86bc46690da926af1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1741650
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63116}

src/objects/js-objects.cc

@@ -4994,13 +4994,9 @@ void JSFunction::InitializeFeedbackCell(Handle<JSFunction> function) {
   Isolate* const isolate = function->GetIsolate();
 
   if (function->has_feedback_vector()) {
-    // TODO(984344): Make this a CHECK that feedback vectors are identical to
-    // what we expect once we have removed all bytecode generation differences
-    // between eager and lazy compilation. For now just reset if they aren't
-    // identical
-    FeedbackVector vector = function->feedback_vector();
-    if (vector.length() == vector.metadata().slot_count()) return;
-    function->raw_feedback_cell().reset();
+    CHECK_EQ(function->feedback_vector().length(),
+             function->feedback_vector().metadata().slot_count());
+    return;
   }
 
   bool needs_feedback_vector = !FLAG_lazy_feedback_allocation;