Re-disallow Memoize for parameterized nested loops with join filters

Re-disallow Memoize for parameterized nested loops with join filters

This was previously fixed in 9e215378d but got broken again as a result
of 2489d76c4.  It seems that commit causes ppi_clauses to contain
duplicate clauses and it's no longer safe to check the list_length of
that list to determine if there are join conditions other than what's
mentioned in ppi_clauses.

Here we adjust the check to count the distinct rinfo_serial mentioned in
ppi_clauses.  We expect that extra->restrictlist won't have duplicate
rinfo_serials.

Reported-by: Amadeo Gallardo
Author: Richard Guo
Discussion: https://postgr.es/m/CADFREbW-BLJd7-a5J%2B5wjVumeFG1ByXiSOFzMtkmY_SDWckTxw%40mail.gmail.com
Backpatch-through: 16, where 2489d76c4 was introduced.