From the AMD side of things, it's just expanding their offerings, and maybe bringing in some know-how and technologies that could help boost/extend their existing products.
If you have Xilinx stocks because you believe in what they're doing and predicting a bright future and possible dividend while at the same time don't see much of a future in the AMD64 platform, then why would you want AMD to the company.
Most stockholders don't care about the companies they invest in any more. It doesn't matter if they make FPGAs, cars or toaster ovens. What matters is that you can make a profit buying a stock now and selling it at a higher place down the road. Those kinds of stockholder won't block an AMD takeover if the price is right.