isl_printer_print_ast_expr: extract out print_ast_expr_c
isl_printer_print_ast_expr: extract out print_ast_expr_c

This will make it easier to introduce an isl format for isl_ast_expr
printing.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,UBERTC/isl,PollyLabs/isl,tobig/isl,nicolasvasilache/isl,inducer/isl-mirror,tobig/isl,inducer/isl-mirror,nicolasvasilache/isl,UBERTC/isl,Meinersbur/isl,nicolasvasilache/isl,PollyLabs/isl,tobig/isl,inducer/isl-mirror,inducer/isl-mirror,UBERTC/isl,tobig/isl,Meinersbur/isl,UBERTC/isl,PollyLabs/isl,nicolasvasilache/isl,PollyLabs/isl,Meinersbur/isl,nicolasvasilache/isl