Vec4<int> and Vec4<float>'s AssignToAll went through the four value
constructor, which stores the values one by one, so whatever loaded the
vector next stalled: the edge functions' setup per triangle and tile,
among others. They now splat in a register on SSE and NEON.
ReadSpanDepth loads its four depths as one vector where they're stored
together, rather than through an array, and ComputePlane is inline, as
returning its plane through memory stalled the UV planes' setup the same
way.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>