Enable use of Memoize atop an Append that came from UNION ALL.

Enable use of Memoize atop an Append that came from UNION ALL.

create_append_path() would only apply get_baserel_parampathinfo
when the path is for a partitioned table, but it's also potentially
useful for paths for UNION ALL appendrels.  Specifically, that
supports building a Memoize path atop this one.

While we're in the vicinity, delete some dead code in
create_merge_append_plan(): there's no need for it to support
parameterized MergeAppend paths, and it doesn't look like that
is going to change anytime soon.  It'll be easy enough to undo
this when/if it becomes useful.

Richard Guo

Discussion: https://postgr.es/m/CAMbWs4_ABSu4PWG2rE1q10tJugEXHWgru3U8dAgkoFvgrb6aEA@mail.gmail.com