Suppress unnecessary explicit sorting for EPQ mergejoin path

When building a ForeignPath for a joinrel, if there's a possibility
that EvalPlanQual will be executed, we must identify a suitable path
for EPQ checks.  If the outer or inner path of the chosen path is a
ForeignPath representing a pushed-down join, we replace it with its
fdw_outerpath to ensure that the EPQ check path consists entirely of
local joins.

If the chosen path is a MergePath, and its outer or inner path is a
ForeignPath that is not already well enough ordered, the MergePath
will have non-NIL outersortkeys or innersortkeys indicating the
desired ordering to be created by an explicit Sort node.  If we then
replace the outer or inner path with its corresponding fdw_outerpath,
and that path is already sufficiently ordered, we end up in an
inconsistent state: the MergePath has non-NIL outersortkeys or
innersortkeys, and its input path is already properly ordered.  This
inconsistency can result in an Assert failure or the addition of a
redundant Sort node.

To fix, check if the new outer or inner path of a MergePath is already
properly sorted, and set its outersortkeys or innersortkeys to NIL if
so.

Bug: #18902
Reported-by: Nikita Kalinin <[email protected]>
Author: Richard Guo <[email protected]>
Reviewed-by: Tender Wang <[email protected]>
Discussion: https://postgr.es/m/[email protected]

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