nir: add an optimization to remove useless phi nodes

nir: add an optimization to remove useless phi nodes

This removes phi nodes whose sources all point to the same thing.

Shader-db results:

total NIR instructions in shared programs: 2045293 -> 2041209 (-0.20%)
NIR instructions in affected programs:     126564 -> 122480 (-3.23%)
helped:                                615
HURT:                                  0

total FS instructions in shared programs: 4321840 -> 4320392 (-0.03%)
FS instructions in affected programs:     24622 -> 23174 (-5.88%)
helped:                                138
HURT:                                  0

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Tested-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>