Revert "Rename files and headers related to index AM"

This follows multiple complains from Peter Geoghegan, Andres Freund and
Alvaro Herrera that this issue ought to be dug more before actually
happening, if it happens.

Discussion: https://postgr.es/m/20191226144606.GA5659@alvherre.pgsql

src/include/access/amvalidate.h

@@ -0,0 +1,36 @@
+/*-------------------------------------------------------------------------
+ *
+ * amvalidate.h
+ *	  Support routines for index access methods' amvalidate functions.
+ *
+ * Copyright (c) 2016-2019, PostgreSQL Global Development Group
+ *
+ * src/include/access/amvalidate.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef AMVALIDATE_H
+#define AMVALIDATE_H
+
+#include "utils/catcache.h"
+
+
+/* Struct returned (in a list) by identify_opfamily_groups() */
+typedef struct OpFamilyOpFuncGroup
+{
+	Oid			lefttype;		/* amoplefttype/amproclefttype */
+	Oid			righttype;		/* amoprighttype/amprocrighttype */
+	uint64		operatorset;	/* bitmask of operators with these types */
+	uint64		functionset;	/* bitmask of support funcs with these types */
+} OpFamilyOpFuncGroup;
+
+
+/* Functions in access/index/amvalidate.c */
+extern List *identify_opfamily_groups(CatCList *oprlist, CatCList *proclist);
+extern bool check_amproc_signature(Oid funcid, Oid restype, bool exact,
+								   int minargs, int maxargs,...);
+extern bool check_amop_signature(Oid opno, Oid restype,
+								 Oid lefttype, Oid righttype);
+extern bool opfamily_can_sort_type(Oid opfamilyoid, Oid datatypeoid);
+
+#endif							/* AMVALIDATE_H */