[X86] Make X86TargetLowering::BuildFILD return a std::pair of SDValues so we...

[X86] Make X86TargetLowering::BuildFILD return a std::pair of SDValues so we explicitly return the chain instead of calling getValue on the single SDValue.

We shouldn't assume that the returned result can be used to get
the other result.

This is prep-work for strict FP where we will also need to pass
the chain result along in more cases.