There is no known (real) hardware with V and without the complete B
extension. B was indeed required in the RISC-V application profile from
2022, earlier than V. There should not be any relevant hardware in the
future either.
In practice, different R-V Vector optimisations in FFmpeg already depend on
every constituent of the B extension anyhow, so it would not work well.
The code was blindly assuming that Zbb or V implied Zba. While the
earlier is practically always true, the later broke some QEMU setups,
as V was introduced earlier than Zba.
To avoid data dependencies, this does the following unroll, which
requires one extra but probably free addition:
coeff = (b * left_weight) >> decorr_shift;
b += a;
a -= coeff;
b -= coeff;
swap(a, b);
