i965: compute DDX in a subspan based only on top row

Consider only the top-left and top-right pixels to approximate DDX in a 2x2
subspan, unless the application requests a more accurate approximation via
GL_FRAGMENT_SHADER_DERIVATIVE_HINT or this optimization is disabled from the
new driconf option disable_derivative_optimization.

This results in a less accurate approximation.  However, it improves the
performance of Xonotic with Ultra settings by 24.3879% +/- 0.832202% (at 95.0%
confidence) on Haswell.  No noticeable image quality difference observed.

The improvement comes from faster sample_d.  It seems, on Haswell, some
optimizations are introduced to allow faster sample_d when all pixels in a
subspan have the same derivative.  I considered SAMPLE_STATE too, which allows
one to control the quality of sample_d on Haswell.  But it gave much worse
image quality without giving better performance comparing to this change.

No piglit quick.tests regression on Haswell, except with in-parameter-struct
and normal-parameter-struct tests which appear to be noises.

Signed-off-by: Chia-I Wu <olv@lunarg.com>
i965/fs: Improve accuracy of dFdy() to match dFdx().

Previously, we computed dFdy() using the following instruction:

  add(8) dst<1>F src<4,4,0)F -src.2<4,4,0>F { align1 1Q }

That had the disadvantage that it computed the same value for all 4
pixels of a 2x2 subspan, which meant that it was less accurate than
dFdx().  This patch changes it to the following instruction when
c->key.high_quality_derivatives is set:

  add(8) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1Q }

This gives it comparable accuracy to dFdx().

Unfortunately, for some reason the SIMD16 version of this instruction:

  add(16) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1H }

Doesn't seem to work reliably (presumably the hardware designers never
validated the combination of align16 mode with compressed
instructions), so we unroll it to:

  add(8) dst<1>F src<4,4,1>.xyxyF -src<4,4,1>.zwzwF { align16 1Q }
  add(8) (dst+1)<1>F (src+1)<4,4,1>.xyxyF -(src+1)<4,4,1>.zwzwF { align16 2Q }

Fixes piglit test spec/glsl-1.10/execution/fs-dfdy-accuracy.