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

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

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

No changes on any other Intel platforms.

Skylake
total instructions in shared programs: 14304261 -> 14304241 (<.01%)
instructions in affected programs: 1625 -> 1605 (-1.23%)
helped: 4
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 5.00 x̃: 5
helped stats (rel) min: 1.01% max: 14.29% x̄: 5.86% x̃: 4.07%
95% mean confidence interval for instructions value: -10.66 0.66
95% mean confidence interval for instructions %-change: -15.91% 4.19%
Inconclusive result (value mean confidence interval includes 0).

total cycles in shared programs: 527531226 -> 527531194 (<.01%)
cycles in affected programs: 92204 -> 92172 (-0.03%)
helped: 2
HURT: 0

Haswell and Broadwell had similar results. (Broadwell shown)
total instructions in shared programs: 14615730 -> 14615710 (<.01%)
instructions in affected programs: 1838 -> 1818 (-1.09%)
helped: 4
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 5.00 x̃: 5
helped stats (rel) min: 0.89% max: 13.04% x̄: 5.37% x̃: 3.78%
95% mean confidence interval for instructions value: -10.66 0.66
95% mean confidence interval for instructions %-change: -14.59% 3.85%
Inconclusive result (value mean confidence interval includes 0).

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