UPDATE 1-Exor offers $6.4 bln to buy PartnerRe, trumping Axis deal

MILAN, April 14 (Reuters) - Italian holding company Exor , which controls carmaker Fiat Chrysler Automobiles , said it had offered $6.4 billion in cash to buy U.S. reinsurer PartnerRe, trumping an earlier all-share deal with Axis.