LibJS: Demote VERIFYs in Completion() to ASSERT

These were *extremely* hot in profiles (noticed when looking at
disassembly).

Now that we've made the special empty JS::Value much harder to create
accidentally, we can feel better about turning these into ASSERT and
catching them in debug builds.

Libraries/LibJS/Runtime/Completion.h

@@ -64,8 +64,8 @@ public:
         : m_type(type)
         , m_value(value)
     {
-        VERIFY(type != Type::Empty);
-        VERIFY(!value.is_special_empty_value());
+        ASSERT(type != Type::Empty);
+        ASSERT(!value.is_special_empty_value());
     }
 
     Completion(ThrowCompletionOr<Value> const&);