Fix yet another issue with step generation in partition pruning.

 Fix yet another issue with step generation in partition pruning.
 
 Commit 13838740f fixed some issues with step generation in partition
 pruning, but there was yet another one: get_steps_using_prefix() assumes
 that clauses in the passed-in prefix list are sorted in ascending order
 of their partition key numbers, but the caller failed to ensure this for
 range partitioning, which led to an assertion failure in debug builds.
 Adjust the caller function to arrange the clauses in the prefix list in
 the required order for range partitioning.
 
 Back-patch to v11, like the previous commit.
 
 Patch by me, reviewed by Amit Langote.
 
 Discussion: https://postgr.es/m/CAPmGK16jkXiFG0YqMbU66wte-oJTfW6D1HaNvQf%3D%2B5o9%3Dm55wQ%40mail.gmail.com