i965/fs: fix compares for doubles

i965/fs: fix compares for doubles

The destination has to have the same source as the type, or else the
simulator will complain. As a result, we need to emit a CMP that
outputs a 64-bit wide result and then do a strided MOV to pick out the
low 32 bits of each channel.

v2: Use subscript() instead of stride() (Curro)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>