LibJS: Clear compile-only data from SharedFunctionInstanceData

After successful bytecode compilation, the m_functions_to_initialize
and m_var_names_to_initialize_binding vectors are no longer needed
as they are only consumed by emit_function_declaration_instantiation()
during code generation.

Add clear_compile_inputs() to release these vectors post-compile,
and call it from both ECMAScriptFunctionObject::get_stack_frame_size()
and NativeJavaScriptBackedFunction::bytecode_executable() after their
respective lazy compilation succeeds.

Also add a pre-compile assertion in Generator::generate_from_function()
to verify we never try to compile the same function data twice, and a
VERIFY in ECMAScriptFunctionObject::ecmascript_code() to guard against
null dereference.

Libraries/LibJS/Runtime/SharedFunctionInstanceData.cpp

@@ -267,4 +267,11 @@ void SharedFunctionInstanceData::visit_edges(Visitor& visitor)
 
 SharedFunctionInstanceData::~SharedFunctionInstanceData() = default;
 
+void SharedFunctionInstanceData::clear_compile_inputs()
+{
+    VERIFY(m_executable);
+    m_functions_to_initialize.clear();
+    m_var_names_to_initialize_binding.clear();
+}
+
 }