[turbofan] Add a DCHECK to FunctionBlueprint comparison

R=mslekova@chromium.org

Bug: v8:7790
Change-Id: I8866ac4a386d535e7c6c00454d93236340ea4fc3
Reviewed-on: https://chromium-review.googlesource.com/c/1477282Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59686}

src/compiler/serializer-for-background-compilation.h

@@ -152,8 +152,10 @@ struct FunctionBlueprint {
   Handle<FeedbackVector> feedback_vector;
 
   bool operator<(const FunctionBlueprint& other) const {
-    // A feedback vector is never used for more than one SFI, so it could
+    // A feedback vector is never used for more than one SFI, so it can
     // be used for strict ordering of blueprints.
+    DCHECK_IMPLIES(feedback_vector.equals(other.feedback_vector),
+                   shared.equals(other.shared));
     return HandleComparator<FeedbackVector>()(feedback_vector,
                                               other.feedback_vector);
   }