pan/mdg: Skip r1.w write where possible

pan/mdg: Skip r1.w write where possible

Should help cycle count. Register pressure is spurious here.

total instructions in shared programs: 50501 -> 49517 (-1.95%)
instructions in affected programs: 33342 -> 32358 (-2.95%)
helped: 393
HURT: 0
helped stats (abs) min: 2 max: 3 x̄: 2.50 x̃: 3
helped stats (rel) min: 0.26% max: 33.33% x̄: 11.99% x̃: 9.09%
95% mean confidence interval for instructions value: -2.55 -2.45
95% mean confidence interval for instructions %-change: -13.01% -10.97%
Instructions are helped.

total bundles in shared programs: 25511 -> 25309 (-0.79%)
bundles in affected programs: 7778 -> 7576 (-2.60%)
helped: 202
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.43% max: 20.00% x̄: 5.97% x̃: 4.35%
95% mean confidence interval for bundles value: -1.00 -1.00
95% mean confidence interval for bundles %-change: -6.65% -5.28%
Bundles are helped.

total quadwords in shared programs: 40789 -> 40339 (-1.10%)
quadwords in affected programs: 25453 -> 25003 (-1.77%)
helped: 273
HURT: 0
helped stats (abs) min: 1 max: 3 x̄: 1.65 x̃: 2
helped stats (rel) min: 0.16% max: 22.22% x̄: 5.99% x̃: 3.92%
95% mean confidence interval for quadwords value: -1.71 -1.59
95% mean confidence interval for quadwords %-change: -6.68% -5.30%
Quadwords are helped.

total registers in shared programs: 3911 -> 3784 (-3.25%)
registers in affected programs: 275 -> 148 (-46.18%)
helped: 129
HURT: 2
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 14.29% max: 50.00% x̄: 48.69% x̃: 50.00%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 50.00% max: 50.00% x̄: 50.00% x̃: 50.00%
95% mean confidence interval for registers value: -1.01 -0.93
95% mean confidence interval for registers %-change: -49.45% -44.91%
Registers are helped.

total threads in shared programs: 2455 -> 2455 (0.00%)
threads in affected programs: 0 -> 0
helped: 0

HURT: 0
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5513>