Processes two channels in parallel, using 128-bit XMM registers.
In theory, we could go up to YMM registers to process 4 channels, but this is
not a gain except for relatively high channel counts (e.g. 7.1), and also
complicates the sample load/store operations considerably.
I decided to only add an AVX variant, since the C code is not substantially
slower enough to justify a separate function just for ancient CPUs.
