Improve index AMs' opclass validation procedures.

The amvalidate functions added in commit 65c5fcd353 were on the
crude side.  Improve them in a few ways:

* Perform signature checking for operators and support functions.

* Apply more thorough checks for missing operators and functions,
where possible.

* Instead of reporting problems as ERRORs, report most problems as INFO
messages and make the amvalidate function return FALSE.  This allows
more than one problem to be discovered per run.

* Report object names rather than OIDs, and work a bit harder on making
the messages understandable.

Also, remove a few more opr_sanity regression test queries that are
now superseded by the amvalidate checks.

src/backend/utils/cache/lsyscache.c

@@ -1102,6 +1102,29 @@ get_opname(Oid opno)
 		return NULL;
 }
 
+/*
+ * get_op_rettype
+ *		Given operator oid, return the operator's result type.
+ */
+Oid
+get_op_rettype(Oid opno)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache1(OPEROID, ObjectIdGetDatum(opno));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_operator optup = (Form_pg_operator) GETSTRUCT(tp);
+		Oid			result;
+
+		result = optup->oprresult;
+		ReleaseSysCache(tp);
+		return result;
+	}
+	else
+		return InvalidOid;
+}
+
 /*
  * op_input_types
  *