i965: Validate "Special Requirements for Handling Double Precision Data Types"

i965: Validate "Special Requirements for Handling Double Precision Data Types"

I did not implement:

   CNL's restriction on 64-bit int + align16, because I don't think
   we'll ever use this combination regardless of hardware generation.

   The restriction on immediate DF -> F conversions, because there's no
   reason to ever generate that, and I don't even know how DF -> F
   conversions are supposed to work in Align16 since (1) the dst stride
   must be 1, but (2) the dst stride would have to be 2 for src and dst
   strides to be aligned.