[Polly][Unroll] Fix unroll_double test.

[Polly][Unroll] Fix unroll_double test.

We enumerated the cross product Domain x Scatter, but sorted only be the
scatter key. In case there are are multiple statement instances per
scatter value, the order between statement instances of the same loop
iteration was undefined.

Propertly enumerate and sort only by the scatter value, and group the
domains using the scatter dimension again.

Thanks to Leonard Chan for the report.