This will serve as a reference for the SIMD backends to come. That said,
with auto-vectorization enabled, the performance of this is not atrocious.
It easily beats the old C code and sometimes even the old SIMD.
In theory, we can dramatically speed it up by using GCC vectors instead of
arrays, but the performance gains from this are too dependent on exact GCC
versions and flags, so it practice it's not a substitute for a SIMD
implementation.