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