freedreno/ir3: Use nir_opt_offset for removing constant adds for shared vars.

Swaps some adds for nops in carchase and manhattan31:

instructions in affected programs: 2746 -> 2768 (0.80%)
nops in affected programs: 1045 -> 1073 (2.68%)
non-nops in affected programs: 1227 -> 1221 (-0.49%)
mov in affected programs: 70 -> 66 (-5.71%)
cat0 in affected programs: 1109 -> 1137 (2.52%)
cat1 in affected programs: 170 -> 166 (-2.35%)
cat2 in affected programs: 142 -> 140 (-1.41%)

just manhattan31/377:

sstall in affected programs: 15 -> 11 (-26.67%)
(ss) in affected programs: 38 -> 37 (-2.63%)
(sy) in affected programs: 16 -> 17 (6.25%)

(Note that this may improve with !14009 (merged))