nir/algebraic: Eliminate useless extract before unpack

nir/algebraic: Eliminate useless extract before unpack

The shader helped for spills and fills is the big compute shader in Dirt
Showdown.  One of the shaders hurt for spills and fills on Broadwell is
the big compute shader in Bioshock Infinite, but combined with the
previous commit, it's still an impovement.

Tiger Lake
total instructions in shared programs: 21833218 -> 21832449 (<.01%)
instructions in affected programs: 66104 -> 65335 (-1.16%)
helped: 106
HURT: 14
helped stats (abs) min: 1 max: 67 x̄: 7.87 x̃: 5
helped stats (rel) min: 0.19% max: 5.76% x̄: 1.27% x̃: 0.95%
HURT stats (abs)   min: 1 max: 14 x̄: 4.64 x̃: 1
HURT stats (rel)   min: 0.19% max: 4.12% x̄: 1.41% x̃: 0.19%
95% mean confidence interval for instructions value: -8.51 -4.30
95% mean confidence interval for instructions %-change: -1.23% -0.69%
Instructions are helped.

total cycles in shared programs: 506180109 -> 506196314 (<.01%)
cycles in affected programs: 1671429 -> 1687634 (0.97%)
helped: 37
HURT: 84
helped stats (abs) min: 1 max: 490 x̄: 73.27 x̃: 24
helped stats (rel) min: 0.02% max: 7.98% x̄: 1.25% x̃: 0.41%
HURT stats (abs)   min: 1 max: 5000 x̄: 225.19 x̃: 8
HURT stats (rel)   min: 0.03% max: 10.22% x̄: 1.22% x̃: 0.42%
95% mean confidence interval for cycles value: 2.85 265.00
95% mean confidence interval for cycles %-change: 0.04% 0.88%
Cycles are HURT.

Ice Lake and Skylake had similar results. (Ice Lake shown)
total instructions in shared programs: 19961317 -> 19960543 (<.01%)
instructions in affected programs: 30268 -> 29494 (-2.56%)
helped: 39
HURT: 0
helped stats (abs) min: 1 max: 142 x̄: 19.85 x̃: 7
helped stats (rel) min: 0.19% max: 7.87% x̄: 2.33% x̃: 2.31%
95% mean confidence interval for instructions value: -29.46 -10.23
95% mean confidence interval for instructions %-change: -2.95% -1.71%
Instructions are helped.

total cycles in shared programs: 498863755 -> 498865843 (<.01%)
cycles in affected programs: 1831136 -> 1833224 (0.11%)
helped: 57
HURT: 65
helped stats (abs) min: 1 max: 1400 x̄: 128.93 x̃: 25
helped stats (rel) min: 0.05% max: 3.49% x̄: 0.89% x̃: 0.71%
HURT stats (abs)   min: 1 max: 1887 x̄: 145.18 x̃: 15
HURT stats (rel)   min: 0.02% max: 9.88% x̄: 1.83% x̃: 0.73%
95% mean confidence interval for cycles value: -58.30 92.53
95% mean confidence interval for cycles %-change: 0.16% 0.97%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 8774 -> 8773 (-0.01%)
spills in affected programs: 20 -> 19 (-5.00%)
helped: 1
HURT: 0

total fills in shared programs: 9496 -> 9494 (-0.02%)
fills in affected programs: 40 -> 38 (-5.00%)
helped: 1
HURT: 0

Broadwell
total instructions in shared programs: 17859373 -> 17858548 (<.01%)
instructions in affected programs: 38452 -> 37627 (-2.15%)
helped: 31
HURT: 0
helped stats (abs) min: 1 max: 143 x̄: 26.61 x̃: 10
helped stats (rel) min: 0.19% max: 7.87% x̄: 2.57% x̃: 2.69%
95% mean confidence interval for instructions value: -39.79 -13.44
95% mean confidence interval for instructions %-change: -3.25% -1.89%
Instructions are helped.

total cycles in shared programs: 525858109 -> 525869236 (<.01%)
cycles in affected programs: 2058597 -> 2069724 (0.54%)
helped: 44
HURT: 75
helped stats (abs) min: 2 max: 1330 x̄: 187.84 x̃: 23
helped stats (rel) min: 0.04% max: 31.31% x̄: 2.13% x̃: 0.85%
HURT stats (abs)   min: 1 max: 3915 x̄: 258.56 x̃: 47
HURT stats (rel)   min: 0.02% max: 10.53% x̄: 2.81% x̃: 2.21%
95% mean confidence interval for cycles value: -26.06 213.07
95% mean confidence interval for cycles %-change: 0.19% 1.78%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 25744 -> 25730 (-0.05%)
spills in affected programs: 1578 -> 1564 (-0.89%)
helped: 4
HURT: 2

total fills in shared programs: 31710 -> 31689 (-0.07%)
fills in affected programs: 4346 -> 4325 (-0.48%)
helped: 3
HURT: 3

Haswell
total instructions in shared programs: 16228399 -> 16227783 (<.01%)
instructions in affected programs: 22201 -> 21585 (-2.77%)
helped: 27
HURT: 0
helped stats (abs) min: 1 max: 68 x̄: 22.81 x̃: 11
helped stats (rel) min: 0.19% max: 7.87% x̄: 2.92% x̃: 2.86%
95% mean confidence interval for instructions value: -31.96 -13.66
95% mean confidence interval for instructions %-change: -3.68% -2.15%
Instructions are helped.

total cycles in shared programs: 538613967 -> 538701354 (0.02%)
cycles in affected programs: 1653044 -> 1740431 (5.29%)
helped: 36
HURT: 81
helped stats (abs) min: 2 max: 708 x̄: 104.50 x̃: 17
helped stats (rel) min: <.01% max: 15.01% x̄: 1.67% x̃: 0.65%
HURT stats (abs)   min: 1 max: 30100 x̄: 1125.30 x̃: 304
HURT stats (rel)   min: 0.02% max: 16.21% x̄: 8.98% x̃: 11.60%
95% mean confidence interval for cycles value: 23.78 1470.01
95% mean confidence interval for cycles %-change: 4.29% 7.12%
Cycles are HURT.

total spills in shared programs: 23418 -> 23409 (-0.04%)
spills in affected programs: 177 -> 168 (-5.08%)
helped: 2
HURT: 0

total fills in shared programs: 25919 -> 25896 (-0.09%)
fills in affected programs: 568 -> 545 (-4.05%)
helped: 3
HURT: 0

Ivy Bridge
total instructions in shared programs: 15265983 -> 15265759 (<.01%)
instructions in affected programs: 8418 -> 8194 (-2.66%)
helped: 5
HURT: 0
helped stats (abs) min: 18 max: 99 x̄: 44.80 x̃: 26
helped stats (rel) min: 1.74% max: 4.26% x̄: 3.12% x̃: 3.00%
95% mean confidence interval for instructions value: -86.29 -3.31
95% mean confidence interval for instructions %-change: -4.43% -1.81%
Instructions are helped.

total cycles in shared programs: 422930336 -> 422929589 (<.01%)
cycles in affected programs: 59347 -> 58600 (-1.26%)
helped: 3
HURT: 2
helped stats (abs) min: 72 max: 1060 x̄: 433.33 x̃: 168
helped stats (rel) min: 1.14% max: 3.48% x̄: 2.23% x̃: 2.06%
HURT stats (abs)   min: 265 max: 288 x̄: 276.50 x̃: 276
HURT stats (rel)   min: 4.79% max: 5.64% x̄: 5.22% x̃: 5.22%
95% mean confidence interval for cycles value: -829.08 530.28
95% mean confidence interval for cycles %-change: -4.43% 5.93%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 4953 -> 4946 (-0.14%)
spills in affected programs: 344 -> 337 (-2.03%)
helped: 2
HURT: 0

total fills in shared programs: 5548 -> 5521 (-0.49%)
fills in affected programs: 838 -> 811 (-3.22%)
helped: 2
HURT: 0

No shader-db changes on any earlier Intel platform.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4515>