I spotted an interesting pattern that I didn't see before that leads to the implementation being faster.
The bit shifting table I was using before is no longer needed, and was able to remove quite a few lines.
I also add use of FMA on the AVX2 version.
f32 1920x1080 1 thread with prelut
c impl
1434012700 UNITS in lut3d->interp, 1 runs, 0 skips
1434035335 UNITS in lut3d->interp, 2 runs, 0 skips
1423615347 UNITS in lut3d->interp, 4 runs, 0 skips
1426268863 UNITS in lut3d->interp, 8 runs, 0 skips
sse2
905484420 UNITS in lut3d->interp, 1 runs, 0 skips
905659010 UNITS in lut3d->interp, 2 runs, 0 skips
915167140 UNITS in lut3d->interp, 4 runs, 0 skips
915834222 UNITS in lut3d->interp, 8 runs, 0 skips
avx
574794860 UNITS in lut3d->interp, 1 runs, 0 skips
581035090 UNITS in lut3d->interp, 2 runs, 0 skips
584116720 UNITS in lut3d->interp, 4 runs, 0 skips
581460290 UNITS in lut3d->interp, 8 runs, 0 skips
avx2
301698880 UNITS in lut3d->interp, 1 runs, 0 skips
301982880 UNITS in lut3d->interp, 2 runs, 0 skips
306962430 UNITS in lut3d->interp, 4 runs, 0 skips
305472025 UNITS in lut3d->interp, 8 runs, 0 skips
gbrap16 1920x1080 1 thread with prelut
c impl
1480894840 UNITS in lut3d->interp, 1 runs, 0 skips
1502922990 UNITS in lut3d->interp, 2 runs, 0 skips
1496114307 UNITS in lut3d->interp, 4 runs, 0 skips
1492554551 UNITS in lut3d->interp, 8 runs, 0 skips
sse2
980777180 UNITS in lut3d->interp, 1 runs, 0 skips
986121520 UNITS in lut3d->interp, 2 runs, 0 skips
986489840 UNITS in lut3d->interp, 4 runs, 0 skips
998832248 UNITS in lut3d->interp, 8 runs, 0 skips
avx
622212360 UNITS in lut3d->interp, 1 runs, 0 skips
622981160 UNITS in lut3d->interp, 2 runs, 0 skips
645396315 UNITS in lut3d->interp, 4 runs, 0 skips
641057075 UNITS in lut3d->interp, 8 runs, 0 skips
avx2
321336400 UNITS in lut3d->interp, 1 runs, 0 skips
321268920 UNITS in lut3d->interp, 2 runs, 0 skips
323459895 UNITS in lut3d->interp, 4 runs, 0 skips
324949967 UNITS in lut3d->interp, 8 runs, 0 skips
