Speeding up gearhash on ARM64 (2× faster)

Automated excerpt

To win on NEON, the dependency chain itself has to get shorter. provided you precompute G = (g₀ << 1) + g₁. Result: 0. 92× → 1. 13×, better, but still well short of the expected 2×. add. 2d v2, v1, v1 ; h << 1 add. 2d v2, v3, v2 ; h₁ = (h<<1) + g₀ shl. 2d v1, v1, #2 ; h << 2 add. 2d v3, v3, v3 ; g₀ << 1 add. 2d v1, v1, v4 ; (h<<2) + g₁ <-- on the h chain add. 2d v1, v3, v1 ; ...

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