pgsql: Match RelOptInfos by relids not pointer equality.

Match RelOptInfos by relids not pointer equality.

Commit 1c2cb2744 added some code that tried to detect whether two
RelOptInfos were the "same" rel by pointer comparison; but it turns
out that inheritance_planner breaks that, through its shenanigans
with copying some relations forward into new subproblems.  Compare
relid sets instead.  Add a regression test case to exercise this
area.

Problem reported by Rushabh Lathia; diagnosis and fix by Amit Langote,
modified a bit by me.

Discussion: https://postgr.es/m/CAGPqQf3anJGj65bqAQ9edDr8gF7qig6_avRgwMT9MsZ19COUPw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/11e22e486d8da6cef4d954c6f2952097df276fa7

Modified Files
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src/backend/partitioning/partprune.c          |  2 +-
src/test/regress/expected/partition_prune.out | 79 +++++++++++++++++++++++++++
src/test/regress/sql/partition_prune.sql      |  6 ++
3 files changed, 86 insertions(+), 1 deletion(-)