i965: Only emit interpolation setup if there are actual FS inputs.

i965: Only emit interpolation setup if there are actual FS inputs.

Dead code elimination would get rid of the extra instructions, but
skipping this saves iterations through the optimization loop.

From shader-db:

      N     Min     Max        Median           Avg        Stddev
x 14672       3      16             3     3.1334515    0.59904168
+ 14672       1      16             3     2.8955153    0.77732963
Difference at 95.0% confidence
        -0.237936 +/- 0.0158798
        -7.59342% +/- 0.506783%
        (Student's t, pooled s = 0.693935)

Embarassingly, the classic shadow mapping shader:

   void main() { }

used to require three iterations through the optimization loop.
With this patch, it only requires one (which makes no progress).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>