cpp_generator::print_method_header: minor clean-up
cpp_generator::print_method_header: minor clean-up

This clean-up will also make it easier to add support for
automatically converting long arguments to isl::val in an upcoming commit.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl