We implement fine derivatives as:

	broadcast(x, (lane & ~1) + 1) - broadcast(x, lane & ~1)

Most of the complexity is to get the right sign. If we can ignore the
sign, we can generate the simpler code:

	broadcast(x, lane ^ 1) - lane

This is a particular win on v7+ where the broadcast instruction (CLPER)
can do `lane ^ value` for free. However, even on v6 where we lower to an
explicit XOR instruction, it's still a win.

The limiting case is fwidth. The fragment shader

   gl_FragColor = fwidth(vec4_varying);

has the following results on v6, v7, and v9:

G72 (-26% instructions, -43% cycles):
38 inst, 30 tuples, 5 clauses, 1.166667 cycles, 1.166667 arith, 28 quadwords
28 inst, 19 tuples, 4 clauses, 0.666667 cycles, 0.666667 arith, 19 quadwords

G76 (-37% instructions, -54% cycles):
38 inst, 30 tuples, 5 clauses, 1.166667 cycles, 1.166667 arith, 28 quadwords
24 inst, 16 tuples, 4 clauses, 0.541667 cycles, 0.541667 arith, 18 quadwords

G78 (-40% instructions, -56% cycles):
40 inst, 1.125000 cycles, 0.250000 fma, 0.109375 cvt, 1.125000 sfu, 20 quadwords
24 inst, 0.500000 cycles, 0.250000 fma, 0.015625 cvt, 0.500000 sfu, 12 quadwords

shader-db tells a similar story -- most shaders are unaffected, but a
shader that uses fwidth has a 20% reduction in cycle count:

instructions helped:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 264 -> 262 (-0.76%)
instructions helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 36 -> 28 (-22.22%)
tuples helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 27 -> 22 (-18.52%)
tuples HURT:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 211 -> 212 (0.47%)
clauses HURT:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 32 -> 33 (3.12%)
cycles helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 1 -> 0.79 (-20.83%)
arith helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 1 -> 0.79 (-20.83%)
quadwords helped:   shaders/chromeos/109-1.shader_test MESA_SHADER_FRAGMENT: 31 -> 28 (-9.68%)
quadwords HURT:   shaders/tesseract/488.shader_test MESA_SHADER_FRAGMENT: 176 -> 178 (1.14%)

total instructions in shared programs: 148370 -> 148360 (<.01%)
instructions in affected programs: 300 -> 290 (-3.33%)
helped: 2
HURT: 0

total tuples in shared programs: 124188 -> 124184 (<.01%)
tuples in affected programs: 238 -> 234 (-1.68%)
helped: 1
HURT: 1
helped stats (abs) min: 5.0 max: 5.0 x̄: 5.00 x̃: 5
helped stats (rel) min: 18.52% max: 18.52% x̄: 18.52% x̃: 18.52%
HURT stats (abs)   min: 1.0 max: 1.0 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.47% max: 0.47% x̄: 0.47% x̃: 0.47%

total clauses in shared programs: 25692 -> 25693 (<.01%)
clauses in affected programs: 32 -> 33 (3.12%)
helped: 0
HURT: 1

total cycles in shared programs: 12132.04 -> 12131.83 (<.01%)
cycles in affected programs: 1 -> 0.79 (-20.83%)
helped: 1
HURT: 0

total arith in shared programs: 4623.75 -> 4623.54 (<.01%)
arith in affected programs: 1 -> 0.79 (-20.83%)
helped: 1
HURT: 0

total quadwords in shared programs: 110386 -> 110385 (<.01%)
quadwords in affected programs: 207 -> 206 (-0.48%)
helped: 1
HURT: 1
helped stats (abs) min: 3.0 max: 3.0 x̄: 3.00 x̃: 3
helped stats (rel) min: 9.68% max: 9.68% x̄: 9.68% x̃: 9.68%
HURT stats (abs)   min: 2.0 max: 2.0 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 1.14% max: 1.14% x̄: 1.14% x̃: 1.14%

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!12332>