ProjectMerge should consider the impure expression

[st1page]

ProjectMerge, which inlines computation and is the reverse of ColumnDedup/CSE. It adds number of calls, while the latter reduces number of calls. 😅

dev=> explain create materialized view mv as  select y, y as y2 from (select abs(x) as y from t);
                                                             QUERY PLAN                                                              
-------------------------------------------------------------------------------------------------------------------------------------
 StreamMaterialize { columns: [y, y2, t._row_id(hidden)], stream_key: [t._row_id], pk_columns: [t._row_id], pk_conflict: "NoCheck" }
 └─StreamProject { exprs: [Abs(t.x) as $expr1, Abs(t.x) as $expr2, t._row_id] }
   └─StreamTableScan { table: t, columns: [x, _row_id] }
(3 rows)

Originally posted by @xxchan in #9133 (comment)

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