nir: Optimize a + neg(a)

nir: Optimize a + neg(a)

Shader-db i965 instructions:
total instructions in shared programs: 1711180 -> 1711159 (-0.00%)
instructions in affected programs:     825 -> 804 (-2.55%)
helped:                                9
HURT:                                  0
GAINED:                                3
LOST:                                  3

Shader-db NIR instructions:
total instructions in shared programs: 606187 -> 606179 (-0.00%)
instructions in affected programs:     298 -> 290 (-2.68%)
helped:                                4
HURT:                                  0
GAINED:                                0
LOST:                                  0

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Thomas Helland <thomashelland90@gmail.com>