pan/bi: Simplify cube map descriptor generation

pan/bi: Simplify cube map descriptor generation

We don't need to do the bitwise manipulation ourselves, we can just use
a bitwise MUX instead.

total instructions in shared programs: 146840 -> 146614 (-0.15%)
instructions in affected programs: 15037 -> 14811 (-1.50%)
helped: 109
HURT: 0
helped stats (abs) min: 2.0 max: 4.0 x̄: 2.07 x̃: 2
helped stats (rel) min: 0.86% max: 4.00% x̄: 1.70% x̃: 1.77%
95% mean confidence interval for instructions value: -2.15 -2.00
95% mean confidence interval for instructions %-change: -1.81% -1.59%
Instructions are helped.

total tuples in shared programs: 128149 -> 128116 (-0.03%)
tuples in affected programs: 2896 -> 2863 (-1.14%)
helped: 16
HURT: 0
helped stats (abs) min: 1.0 max: 5.0 x̄: 2.06 x̃: 1
helped stats (rel) min: 0.65% max: 2.33% x̄: 1.16% x̃: 0.70%
95% mean confidence interval for tuples value: -3.01 -1.12
95% mean confidence interval for tuples %-change: -1.50% -0.83%
Tuples are helped.

total cycles in shared programs: 12257.10 -> 12250.81 (-0.05%)
cycles in affected programs: 449.87 -> 443.58 (-1.40%)
helped: 92
HURT: 0
helped stats (abs) min: 0.0416660000000002 max: 0.20833400000000069 x̄: 0.07 x̃: 0
helped stats (rel) min: 0.93% max: 2.53% x̄: 1.40% x̃: 1.26%
95% mean confidence interval for cycles value: -0.08 -0.06
95% mean confidence interval for cycles %-change: -1.48% -1.32%
Cycles are helped.

total arith in shared programs: 4847.33 -> 4840.25 (-0.15%)
arith in affected programs: 490.37 -> 483.29 (-1.44%)
helped: 109
HURT: 0
helped stats (abs) min: 0.0416660000000002 max: 0.20833400000000069 x̄: 0.06 x̃: 0
helped stats (rel) min: 0.93% max: 5.56% x̄: 1.51% x̃: 1.26%
95% mean confidence interval for arith value: -0.07 -0.06
95% mean confidence interval for arith %-change: -1.64% -1.39%
Arith are helped.

total quadwords in shared programs: 116775 -> 116758 (-0.01%)
quadwords in affected programs: 1331 -> 1314 (-1.28%)
helped: 7
HURT: 0
helped stats (abs) min: 1.0 max: 4.0 x̄: 2.43 x̃: 3
helped stats (rel) min: 0.91% max: 2.38% x̄: 1.65% x̃: 1.39%
95% mean confidence interval for quadwords value: -3.48 -1.38
95% mean confidence interval for quadwords %-change: -2.27% -1.04%
Quadwords are helped.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11733>