Instead of computing y + N with a hard-coded index offset, calculate the
relative offset as a 16-bit integer in C and add that to the pointer directly.
Since we no longer mask the resulting combined address, this may result in
overread, but that's fine since we over-provisioned the array in the previous
commit.
This is an exceptionally unlikely (in fact, currently impossible) case to
actually hit, and not worth micro-optimizing for. More specifically, having
this special case prevents me from easily adding per-row offsets.
This covers most 8-bit and 16-bit ops, and some 32-bit ops. It also covers all
floating point operations. While this is not yet 100% coverage, it's good
enough for the vast majority of formats out there.
Of special note is the packed shuffle fast path, which uses pshufb at vector
sizes up to AVX512.
