pan/mdg: Canonicalize (x * 2.0) to (x + x)

pan/mdg: Canonicalize (x * 2.0) to (x + x)

This lets the previous commit kick in to schedule to either a multiply
or an add. GLES2 shader-db:

total instructions in shared programs: 50514 -> 50459 (-0.11%)
instructions in affected programs: 7436 -> 7381 (-0.74%)
helped: 14
HURT: 7
helped stats (abs) min: 2 max: 8 x̄: 5.00 x̃: 5
helped stats (rel) min: 0.95% max: 1.14% x̄: 1.07% x̃: 1.08%
HURT stats (abs)   min: 2 max: 3 x̄: 2.14 x̃: 2
HURT stats (rel)   min: 0.85% max: 8.57% x̄: 2.73% x̃: 1.26%
95% mean confidence interval for instructions value: -4.37 -0.87
95% mean confidence interval for instructions %-change: -0.91% 1.31%
Inconclusive result (%-change mean confidence interval includes 0).

total bundles in shared programs: 25680 -> 25573 (-0.42%)
bundles in affected programs: 6148 -> 6041 (-1.74%)
helped: 37
HURT: 7
helped stats (abs) min: 1 max: 9 x̄: 3.14 x̃: 2
helped stats (rel) min: 0.63% max: 8.33% x̄: 2.02% x̃: 2.13%
HURT stats (abs)   min: 1 max: 2 x̄: 1.29 x̃: 1
HURT stats (rel)   min: 0.88% max: 11.11% x̄: 3.92% x̃: 1.30%
95% mean confidence interval for bundles value: -3.32 -1.54
95% mean confidence interval for bundles %-change: -2.00% -0.14%
Bundles are helped.

total quadwords in shared programs: 40887 -> 40815 (-0.18%)
quadwords in affected programs: 14203 -> 14131 (-0.51%)
helped: 61
HURT: 2
helped stats (abs) min: 1 max: 4 x̄: 1.21 x̃: 1
helped stats (rel) min: 0.16% max: 11.11% x̄: 1.11% x̃: 0.57%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 2.86% max: 4.00% x̄: 3.43% x̃: 3.43%
95% mean confidence interval for quadwords value: -1.32 -0.96
95% mean confidence interval for quadwords %-change: -1.46% -0.48%
Quadwords are helped.

total registers in shared programs: 3916 -> 3913 (-0.08%)
registers in affected programs: 46 -> 43 (-6.52%)
helped: 5
HURT: 1
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 10.00% max: 33.33% x̄: 14.89% x̃: 10.00%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 50.00% max: 50.00% x̄: 50.00% x̃: 50.00%
95% mean confidence interval for registers value: -1.79 0.79
95% mean confidence interval for registers %-change: -33.51% 25.37%
Inconclusive result (value mean confidence interval includes 0).

total threads in shared programs: 2455 -> 2454 (-0.04%)
threads in affected programs: 5 -> 4 (-20.00%)
helped: 1
HURT: 1
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 100.00% max: 100.00% x̄: 100.00% x̃: 100.00%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 50.00% max: 50.00% x̄: 50.00% x̃: 50.00%

total loops in shared programs: 6 -> 6 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total spills in shared programs: 168 -> 168 (0.00%)
spills in affected programs: 0 -> 0
helped: 0
HURT: 0

total fills in shared programs: 186 -> 186 (0.00%)
fills in affected programs: 0 -> 0
helped: 0
HURT: 0

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5475>