isl_printer_print_map: fix printing of affine expressions in tuples

isl_printer_print_map: fix printing of affine expressions in tuples

The set { [1, y] : 0 <= y <= 1; [x, -x] : 0 <= x <= 1 }
would get printed as { [1, y] : 0 <= y <= 1; [i0, y = -x] : 0 <= i0 <= 1 }.
Arguably, the problem has been around since efd6c5e (support nested isl_dims,
Mon Aug 2 16:56:35 2010 +0200), since it would use different spaces
for printing identifiers and affine expressions, although the different
spaces may in the end always have been the same.
It became a real problem in e923e3d (isl_printer_print_map: use space from
outer map for variable names, Thu Jan 7 12:27:59 2016 +0100) since
it could cause those two spaces to be different, resulting in the set
above getting printed as { [1, y] : 0 <= y <= 1; [i0, -x] : 0 <= i0 <= 1 }.

Use the same space for printing identifiers and printing affine expressions.

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