pan/bi: Lower fragment output with <4 components

pan/bi: Lower fragment output with <4 components

This avoids undefined behaviour in the shader, which will fail
validation added later in the series. shader-db results are neglible --
the extra moves required in a few cases are cancelled out by the extra
moves eliminated by allowing register allocation to work properly.

total instructions in shared programs: 146903 -> 146907 (<.01%)
instructions in affected programs: 33 -> 37 (12.12%)
helped: 0
HURT: 1

total tuples in shared programs: 123616 -> 123613 (<.01%)
tuples in affected programs: 764 -> 761 (-0.39%)
helped: 6
HURT: 4
helped stats (abs) min: 1.0 max: 4.0 x̄: 1.67 x̃: 1
helped stats (rel) min: 0.54% max: 5.88% x̄: 2.64% x̃: 1.86%
HURT stats (abs)   min: 1.0 max: 2.0 x̄: 1.75 x̃: 2
HURT stats (rel)   min: 4.55% max: 13.33% x̄: 8.57% x̃: 8.19%
95% mean confidence interval for tuples value: -1.73 1.13
95% mean confidence interval for tuples %-change: -2.72% 6.41%
Inconclusive result (value mean confidence interval includes 0).

total clauses in shared programs: 25656 -> 25654 (<.01%)
clauses in affected programs: 43 -> 41 (-4.65%)
helped: 2
HURT: 1
helped stats (abs) min: 1.0 max: 2.0 x̄: 1.50 x̃: 1
helped stats (rel) min: 6.25% max: 12.50% x̄: 9.38% x̃: 9.38%
HURT stats (abs)   min: 1.0 max: 1.0 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 33.33% max: 33.33% x̄: 33.33% x̃: 33.33%

total cycles in shared programs: 12114.21 -> 12114.12 (<.01%)
cycles in affected programs: 27.42 -> 27.33 (-0.30%)
helped: 4
HURT: 3
helped stats (abs) min: 0.04166700000000034 max: 0.08333299999999966 x̄: 0.06 x̃: 0
helped stats (rel) min: 0.57% max: 1.59% x̄: 1.02% x̃: 0.96%
HURT stats (abs)   min: 0.0416669999999999 max: 0.08333299999999999 x̄: 0.06 x̃: 0
HURT stats (rel)   min: 4.17% max: 16.67% x̄: 8.80% x̃: 5.56%
95% mean confidence interval for cycles value: -0.07 0.05
95% mean confidence interval for cycles %-change: -2.90% 9.27%
Inconclusive result (value mean confidence interval includes 0).

total arith in shared programs: 4601.08 -> 4601.04 (<.01%)
arith in affected programs: 29 -> 28.96 (-0.14%)
helped: 6
HURT: 4
helped stats (abs) min: 0.04166700000000001 max: 0.08333299999999966 x̄: 0.06 x̃: 0
helped stats (rel) min: 0.57% max: 10.00% x̄: 3.63% x̃: 1.39%
HURT stats (abs)   min: 0.04166700000000001 max: 0.08333399999999991 x̄: 0.07 x̃: 0
HURT stats (rel)   min: 5.56% max: 16.67% x̄: 10.85% x̃: 10.60%
95% mean confidence interval for arith value: -0.05 0.05
95% mean confidence interval for arith %-change: -3.95% 8.28%
Inconclusive result (value mean confidence interval includes 0).

total quadwords in shared programs: 110008 -> 110002 (<.01%)
quadwords in affected programs: 1090 -> 1084 (-0.55%)
helped: 11
HURT: 8
helped stats (abs) min: 1.0 max: 7.0 x̄: 2.18 x̃: 1
helped stats (rel) min: 0.61% max: 13.16% x̄: 4.07% x̃: 1.82%
HURT stats (abs)   min: 1.0 max: 6.0 x̄: 2.25 x̃: 1
HURT stats (rel)   min: 3.70% max: 42.86% x̄: 12.55% x̃: 7.50%
95% mean confidence interval for quadwords value: -1.76 1.13
95% mean confidence interval for quadwords %-change: -2.95% 8.81%
Inconclusive result (value mean confidence interval includes 0).

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12130>