isl_multi_*_set_dim_name: use isl_multi_*_reset_space
isl_multi_*_set_dim_name: use isl_multi_*_reset_space

This simplifies the code, mimicking the isl_multi_*_set_dim_id
implementation.

Note that isl_multi_*_reset_space was not available
when isl_multi_*_set_dim_name was introduced in isl-0.07-194-g3d0da01fea
(add isl_multi_aff, Thu Sep 1 11:49:19 2011 +0200).

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

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