extract generic isl_*_list_foreach_scc from isl_ast_codegen.c

extract generic isl_*_list_foreach_scc from isl_ast_codegen.c

isl_ast_codegen.c contained two implementations of this function
that have now been replaced by two calls to the extracted functions.
This makes isl_ast_codegen.c less dependent on the internals of
isl_basic_set_list.  We should also be able to later reuse the
extracted function in other contexts.  Once all users of isl_tarjan
have been converted to this interface, we probably want to remove
the old interface and perform the calls to callback as soon as
each SCC is found rather than waiting until all of them have been found.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>