[Impeller] sped up the linear gradient by moving calculations to the cpu

[gaaclarke]

issue: flutter/flutter#148496

The linked issue showed us as ALU bound with the linear gradient, this should relieve some of that pressure.

Tests: There is already existing golden tests for this, this just changed the performance, no new functionality.

summary of math improvements:

• (num_colors - 1) * division -> multiplication
• (num_colors - 1) * subtractions removed
• removes 1 dot()
• 1 division -> multiplication
• 1 subtraction removed

Pre-launch Checklist

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[gaaclarke]

I ran out of time today, I'll try to profile this tomorrow to get a sense of what kind of difference this makes.

[gaaclarke]

Malioc diff:

In variant Main:
  fp16_arithmetic: 35 <- before
  fp16_arithmetic: 37 <- after
total_cycles in variant Main
        arith_total arith_fma   arith_cvt   arith_sfu   load_store  varying     texture     bound       
before  0.750000    0.421875    0.750000    0.625000    4.000000    0.250000    0.000000    load_store
after   0.718750    0.406250    0.718750    0.562500    4.000000    0.250000    0.000000    load_store

In variant Main:
  work_registers_used: 17 <- before
  work_registers_used: 21 <- after

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Changes reported for pull request #53007 at sha 9eeb55d

[jonahwilliams]

Fuck yeah, optimizations!

LGTM

[gaaclarke]

Here's a rough estimate of the difference on my iphone se 2022. These are singleton datapoints so your mileage may vary.

before

after