isl_reordering_extend: use target length of isl_reordering
isl_reordering_extend: use target length of isl_reordering

This simplifies the code.
Note that the original code assumed that no isl_reordering_extend
had been called on the isl_reordering before, while the new code
would also allow isl_reordering_extend to be applied several times,
even though this is not likely to ever be needed.

Reviewed-by: Tianjiao Sun <07f9c69eb0c44438de986fa0c33d1e083f209b1b@cerebras.net>
Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

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