LibJS: Make GetById and GetByValue avoid get_identifier() in common case

We now defer looking up the various identifiers by IdentifierTableIndex
until the last moment. This allows us to avoid the retrieval in common
cases like when a property access is cached.

Knocks a ~12% item off the profile on https://ventrella.com/Clusters/

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -402,6 +402,8 @@ private:
     bool m_must_propagate_completion { true };
 
     GCPtr<ECMAScriptFunctionObject const> m_function;
+
+    Optional<IdentifierTableIndex> m_length_identifier;
 };
 
 }