nir: detect more dynamically uniform expressions

nir: detect more dynamically uniform expressions

Shader-db results for v3d:

total instructions in shared programs: 9132728 -> 9119238 (-0.15%)
instructions in affected programs: 596886 -> 583396 (-2.26%)
helped: 1118
HURT: 224

total threads in shared programs: 234298 -> 234308 (<.01%)
threads in affected programs: 10 -> 20 (100.00%)
helped: 5
HURT: 0

total uniforms in shared programs: 3022949 -> 3022622 (-0.01%)
uniforms in affected programs: 29163 -> 28836 (-1.12%)
helped: 108
HURT: 37

total max-temps in shared programs: 1328030 -> 1327762 (-0.02%)
max-temps in affected programs: 10097 -> 9829 (-2.65%)
helped: 263
HURT: 15

total spills in shared programs: 3793 -> 3777 (-0.42%)
spills in affected programs: 432 -> 416 (-3.70%)
helped: 16
HURT: 0

total fills in shared programs: 4380 -> 4266 (-2.60%)
fills in affected programs: 828 -> 714 (-13.77%)
helped: 16
HURT: 0

Reviewed-by: Eric Anholt <eric@anholt.net>