nir/algebraic: Reassociate shift-by-constant of shift-by-constant

nir/algebraic: Reassociate shift-by-constant of shift-by-constant

v2: After some review discussion with Alyssa, the replacements now
correct account for cases where (b+c) >= bitsize.

v3: Use a temporary to simplify the Python code quite a bit.  Suggested
by Jason.

Haswell and all Gen8+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 16251155 -> 16249576 (<.01%)
instructions in affected programs: 232627 -> 231048 (-0.68%)
helped: 547
HURT: 1
helped stats (abs) min: 1 max: 15 x̄: 2.89 x̃: 3
helped stats (rel) min: 0.04% max: 7.84% x̄: 1.14% x̃: 1.06%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.12% max: 0.12% x̄: 0.12% x̃: 0.12%
95% mean confidence interval for instructions value: -3.12 -2.65
95% mean confidence interval for instructions %-change: -1.20% -1.06%
Instructions are helped.

total cycles in shared programs: 365924392 -> 365372103 (-0.15%)
cycles in affected programs: 59207053 -> 58654764 (-0.93%)
helped: 497
HURT: 34
helped stats (abs) min: 1 max: 29300 x̄: 1118.16 x̃: 16
helped stats (rel) min: <.01% max: 10.59% x̄: 1.82% x̃: 1.82%
HURT stats (abs)   min: 2 max: 424 x̄: 101.03 x̃: 63
HURT stats (rel)   min: 0.07% max: 46.17% x̄: 4.72% x̃: 2.06%
95% mean confidence interval for cycles value: -1426.41 -653.77
95% mean confidence interval for cycles %-change: -1.66% -1.15%
Cycles are helped.

total spills in shared programs: 8870 -> 8871 (0.01%)
spills in affected programs: 104 -> 105 (0.96%)
helped: 0
HURT: 1

Ivy Bridge and all pre-Gen7 platforms had similar results. (Ivy Bridge shown)
total instructions in shared programs: 11956236 -> 11955635 (<.01%)
instructions in affected programs: 94110 -> 93509 (-0.64%)
helped: 106
HURT: 0
helped stats (abs) min: 1 max: 14 x̄: 5.67 x̃: 4
helped stats (rel) min: 0.12% max: 4.71% x̄: 1.96% x̃: 0.76%
95% mean confidence interval for instructions value: -6.62 -4.72
95% mean confidence interval for instructions %-change: -2.27% -1.64%
Instructions are helped.

total cycles in shared programs: 179296340 -> 178788044 (-0.28%)
cycles in affected programs: 51009603 -> 50501307 (-1.00%)
helped: 82
HURT: 7
helped stats (abs) min: 5 max: 27820 x̄: 6199.00 x̃: 16
helped stats (rel) min: 0.30% max: 8.16% x̄: 2.58% x̃: 3.11%
HURT stats (abs)   min: 2 max: 8 x̄: 3.14 x̃: 2
HURT stats (rel)   min: 0.02% max: 1.40% x̄: 0.34% x̃: 0.10%
95% mean confidence interval for cycles value: -7649.38 -3773.00
95% mean confidence interval for cycles %-change: -2.71% -1.99%
Cycles are helped.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> [v2]
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>