nir: Turn imov/fmov of undef into undef.

nir: Turn imov/fmov of undef into undef.

On Broadwell:

total instructions in shared programs: 11640214 -> 11639872 (-0.00%)
instructions in affected programs: 17744 -> 17402 (-1.93%)
helped: 78
HURT: 0

total spills in shared programs: 2924 -> 2922 (-0.07%)
spills in affected programs: 104 -> 102 (-1.92%)
helped: 1
HURT: 0

total fills in shared programs: 4394 -> 4389 (-0.11%)
fills in affected programs: 237 -> 232 (-2.11%)
helped: 1
HURT: 0

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>