LibJS: Avoid IdentifierTable lookup in cached GetGlobal op

When we hit the cache in GetGlobal, we don't need the identifier string
at all, so let's defer fetching it until after the cache miss.

7% speed-up on Kraken/imaging-gaussian-blur.js :^)

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -880,8 +880,6 @@ ThrowCompletionOr<void> GetGlobal::execute_impl(Bytecode::Interpreter& interpret
     auto& vm = interpreter.vm();
     auto& realm = *vm.current_realm();
 
-    auto const& name = interpreter.current_executable().get_identifier(m_identifier);
-
     auto& cache = interpreter.current_executable().global_variable_caches[m_cache_index];
     auto& binding_object = realm.global_environment().object_record().binding_object();
     auto& declarative_record = realm.global_environment().declarative_record();
@@ -898,6 +896,7 @@ ThrowCompletionOr<void> GetGlobal::execute_impl(Bytecode::Interpreter& interpret
 
     cache.environment_serial_number = declarative_record.environment_serial_number();
 
+    auto const& name = interpreter.current_executable().get_identifier(m_identifier);
     if (vm.running_execution_context().script_or_module.has<NonnullGCPtr<Module>>()) {
         // NOTE: GetGlobal is used to access variables stored in the module environment and global environment.
         //       The module environment is checked first since it precedes the global environment in the environment chain.