i965/fs: Avoid constant propagation when the type sizes don't match.

i965/fs: Avoid constant propagation when the type sizes don't match.

The case where the source type of the instruction is smaller than the
immediate type could be handled by calculating the portion of the
immediate read by the instruction (assuming that the source channels
are aligned with the destination channels of the copy) and then
representing the same value as an immediate of the source type
(assuming such an immediate type exists), but the code below doesn't
do that, so just bail for the moment.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>