Move inheritance expansion code into its own file

This commit moves expand_inherited_tables and underlings from
optimizer/prep/prepunionc.c to optimizer/utils/inherit.c.
Also, all of the AppendRelInfo-based expression manipulation routines
are moved to optimizer/utils/appendinfo.c.

No functional code changes.  One exception is the introduction of
make_append_rel_info, but that's still just moving around code.

Also, stop including <limits.h> in prepunion.c, which no longer needs
it since 3fc6e2d.  I (Álvaro) noticed this because Amit was copying
that to inherit.c, which likewise doesn't need it.

Author: Amit Langote
Discussion: https://postgr.es/m/3be67028-a00a-502c-199a-da00eec8fb6e@lab.ntt.co.jp

Author: alvherre

src/backend/optimizer/path/allpaths.c

@@ -30,14 +30,15 @@
 #ifdef OPTIMIZER_DEBUG
 #include "nodes/print.h"
 #endif
+#include "optimizer/appendinfo.h"
 #include "optimizer/clauses.h"
 #include "optimizer/cost.h"
 #include "optimizer/geqo.h"
+#include "optimizer/inherit.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "optimizer/plancat.h"
 #include "optimizer/planner.h"
-#include "optimizer/prep.h"
 #include "optimizer/restrictinfo.h"
 #include "optimizer/tlist.h"
 #include "optimizer/var.h"

src/backend/optimizer/path/equivclass.c

@@ -22,11 +22,11 @@
 #include "catalog/pg_type.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
+#include "optimizer/appendinfo.h"
 #include "optimizer/clauses.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "optimizer/planmain.h"
-#include "optimizer/prep.h"
 #include "optimizer/restrictinfo.h"
 #include "optimizer/var.h"
 #include "utils/lsyscache.h"

src/backend/optimizer/path/joinrels.c

@@ -15,11 +15,11 @@
 #include "postgres.h"
 
 #include "miscadmin.h"
+#include "optimizer/appendinfo.h"
 #include "optimizer/clauses.h"
 #include "optimizer/joininfo.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
-#include "optimizer/prep.h"
 #include "partitioning/partbounds.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"

src/backend/optimizer/plan/planmain.c

@@ -20,7 +20,9 @@
  */
 #include "postgres.h"
 
+#include "optimizer/appendinfo.h"
 #include "optimizer/clauses.h"
+#include "optimizer/inherit.h"
 #include "optimizer/orclauses.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"

src/backend/optimizer/plan/planner.c

@@ -37,8 +37,10 @@
 #ifdef OPTIMIZER_DEBUG
 #include "nodes/print.h"
 #endif
+#include "optimizer/appendinfo.h"
 #include "optimizer/clauses.h"
 #include "optimizer/cost.h"
+#include "optimizer/inherit.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "optimizer/plancat.h"