LibJS/JIT: Add fast path for GetById of Array.length

Array.length is magical (since it has to reflect the number of elements
in the object's property storage).

We now handle it specially in jitted code, giving us a massive speed-up
on Kraken/ai-astar.js (and probably many other things as well) :^)

Userland/Libraries/LibJS/Runtime/Array.cpp

@@ -64,6 +64,7 @@ NonnullGCPtr<Array> Array::create_from(Realm& realm, Vector<Value> const& elemen
 Array::Array(Object& prototype)
     : Object(ConstructWithPrototypeTag::Tag, prototype)
 {
+    m_has_magical_length_property = true;
 }
 
 // 10.4.2.4 ArraySetLength ( A, Desc ), https://tc39.es/ecma262/#sec-arraysetlength