i965/fs: Emit BRW_AOP_INC or BRW_AOP_DEC for atomicAdd of +1 or -1

i965/fs: Emit BRW_AOP_INC or BRW_AOP_DEC for atomicAdd of +1 or -1

Funny story... a single shader was hurt for instructions, spills, fills.
That same shader was also the most helped for cycles.  #GPUsAreWeird

No changes on any other Intel platform.

v2: Refactor selection of atomic opcode to a separate function.
Suggested by Jason.

Haswell, Broadwell, and Skylake had similar results. (Skylake shown)
total instructions in shared programs: 14304116 -> 14304261 (<.01%)
instructions in affected programs: 12776 -> 12921 (1.13%)
helped: 19
HURT: 1
helped stats (abs) min: 1 max: 16 x̄: 2.32 x̃: 1
helped stats (rel) min: 0.05% max: 7.27% x̄: 0.92% x̃: 0.55%
HURT stats (abs)   min: 189 max: 189 x̄: 189.00 x̃: 189
HURT stats (rel)   min: 4.87% max: 4.87% x̄: 4.87% x̃: 4.87%
95% mean confidence interval for instructions value: -12.83 27.33
95% mean confidence interval for instructions %-change: -1.57% 0.31%
Inconclusive result (value mean confidence interval includes 0).

total cycles in shared programs: 527552861 -> 527531226 (<.01%)
cycles in affected programs: 1459195 -> 1437560 (-1.48%)
helped: 16
HURT: 2
helped stats (abs) min: 2 max: 21328 x̄: 1353.69 x̃: 6
helped stats (rel) min: 0.01% max: 5.29% x̄: 0.36% x̃: 0.03%
HURT stats (abs)   min: 12 max: 12 x̄: 12.00 x̃: 12
HURT stats (rel)   min: 0.03% max: 0.03% x̄: 0.03% x̃: 0.03%
95% mean confidence interval for cycles value: -3699.81 1295.92
95% mean confidence interval for cycles %-change: -0.94% 0.30%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 8025 -> 8033 (0.10%)
spills in affected programs: 208 -> 216 (3.85%)
helped: 1
HURT: 1

total fills in shared programs: 10989 -> 11040 (0.46%)
fills in affected programs: 444 -> 495 (11.49%)
helped: 1
HURT: 1

Ivy Bridge
total instructions in shared programs: 11709181 -> 11709153 (<.01%)
instructions in affected programs: 3505 -> 3477 (-0.80%)
helped: 3
HURT: 0
helped stats (abs) min: 1 max: 23 x̄: 9.33 x̃: 4
helped stats (rel) min: 0.11% max: 1.16% x̄: 0.63% x̃: 0.61%

total cycles in shared programs: 254741126 -> 254738801 (<.01%)
cycles in affected programs: 919067 -> 916742 (-0.25%)
helped: 3
HURT: 0
helped stats (abs) min: 21 max: 2144 x̄: 775.00 x̃: 160
helped stats (rel) min: 0.03% max: 0.90% x̄: 0.32% x̃: 0.03%

total spills in shared programs: 4536 -> 4533 (-0.07%)
spills in affected programs: 40 -> 37 (-7.50%)
helped: 1
HURT: 0

total fills in shared programs: 4819 -> 4813 (-0.12%)
fills in affected programs: 94 -> 88 (-6.38%)
helped: 1
HURT: 0

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com> [v1]
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>