Make some small planner API cleanups.

Move a few very simple node-creation and node-type-testing functions
from the planner's clauses.c to nodes/makefuncs and nodes/nodeFuncs.
There's nothing planner-specific about them, as evidenced by the
number of other places that were using them.

While at it, rename and_clause() etc to is_andclause() etc, to clarify
that they are node-type-testing functions not node-creation functions.
And use "static inline" implementations for the shortest ones.

Also, modify flatten_join_alias_vars() and some subsidiary functions
to take a Query not a PlannerInfo to define the join structure that
Vars should be translated according to.  They were only using the
"parse" field of the PlannerInfo anyway, so this just requires removing
one level of indirection.  The advantage is that now parse_agg.c can
use flatten_join_alias_vars() without the horrid kluge of creating an
incomplete PlannerInfo, which will allow that file to be decoupled from
relation.h in a subsequent patch.

Discussion: https://postgr.es/m/11460.1548706639@sss.pgh.pa.us

src/backend/nodes/makefuncs.c

@@ -598,6 +598,141 @@ makeFuncCall(List *name, List *args, int location)
 	return n;
 }
 
+/*
+ * make_opclause
+ *	  Creates an operator clause given its operator info, left operand
+ *	  and right operand (pass NULL to create single-operand clause),
+ *	  and collation info.
+ */
+Expr *
+make_opclause(Oid opno, Oid opresulttype, bool opretset,
+			  Expr *leftop, Expr *rightop,
+			  Oid opcollid, Oid inputcollid)
+{
+	OpExpr	   *expr = makeNode(OpExpr);
+
+	expr->opno = opno;
+	expr->opfuncid = InvalidOid;
+	expr->opresulttype = opresulttype;
+	expr->opretset = opretset;
+	expr->opcollid = opcollid;
+	expr->inputcollid = inputcollid;
+	if (rightop)
+		expr->args = list_make2(leftop, rightop);
+	else
+		expr->args = list_make1(leftop);
+	expr->location = -1;
+	return (Expr *) expr;
+}
+
+/*
+ * make_andclause
+ *
+ * Creates an 'and' clause given a list of its subclauses.
+ */
+Expr *
+make_andclause(List *andclauses)
+{
+	BoolExpr   *expr = makeNode(BoolExpr);
+
+	expr->boolop = AND_EXPR;
+	expr->args = andclauses;
+	expr->location = -1;
+	return (Expr *) expr;
+}
+
+/*
+ * make_orclause
+ *
+ * Creates an 'or' clause given a list of its subclauses.
+ */
+Expr *
+make_orclause(List *orclauses)
+{
+	BoolExpr   *expr = makeNode(BoolExpr);
+
+	expr->boolop = OR_EXPR;
+	expr->args = orclauses;
+	expr->location = -1;
+	return (Expr *) expr;
+}
+
+/*
+ * make_notclause
+ *
+ * Create a 'not' clause given the expression to be negated.
+ */
+Expr *
+make_notclause(Expr *notclause)
+{
+	BoolExpr   *expr = makeNode(BoolExpr);
+
+	expr->boolop = NOT_EXPR;
+	expr->args = list_make1(notclause);
+	expr->location = -1;
+	return (Expr *) expr;
+}
+
+/*
+ * make_and_qual
+ *
+ * Variant of make_andclause for ANDing two qual conditions together.
+ * Qual conditions have the property that a NULL nodetree is interpreted
+ * as 'true'.
+ *
+ * NB: this makes no attempt to preserve AND/OR flatness; so it should not
+ * be used on a qual that has already been run through prepqual.c.
+ */
+Node *
+make_and_qual(Node *qual1, Node *qual2)
+{
+	if (qual1 == NULL)
+		return qual2;
+	if (qual2 == NULL)
+		return qual1;
+	return (Node *) make_andclause(list_make2(qual1, qual2));
+}
+
+/*
+ * The planner and executor usually represent qualification expressions
+ * as lists of boolean expressions with implicit AND semantics.
+ *
+ * These functions convert between an AND-semantics expression list and the
+ * ordinary representation of a boolean expression.
+ *
+ * Note that an empty list is considered equivalent to TRUE.
+ */
+Expr *
+make_ands_explicit(List *andclauses)
+{
+	if (andclauses == NIL)
+		return (Expr *) makeBoolConst(true, false);
+	else if (list_length(andclauses) == 1)
+		return (Expr *) linitial(andclauses);
+	else
+		return make_andclause(andclauses);
+}
+
+List *
+make_ands_implicit(Expr *clause)
+{
+	/*
+	 * NB: because the parser sets the qual field to NULL in a query that has
+	 * no WHERE clause, we must consider a NULL input clause as TRUE, even
+	 * though one might more reasonably think it FALSE.
+	 */
+	if (clause == NULL)
+		return NIL;				/* NULL -> NIL list == TRUE */
+	else if (is_andclause(clause))
+		return ((BoolExpr *) clause)->args;
+	else if (IsA(clause, Const) &&
+			 !((Const *) clause)->constisnull &&
+			 DatumGetBool(((Const *) clause)->constvalue))
+		return NIL;				/* constant TRUE input -> NIL list */
+	else
+		return list_make1(clause);
+}
+
 /*
  * makeGroupingSet
  *