In this case, the inner loop computing the scalar product can be reduced
to just one multiplication and one sum even with 128-bit vectors. The
result is a lot simpler, but also brings more modest performance gains:

flac_lpc_16_13_c:       15241.0
flac_lpc_16_13_rvv_i32: 11230.0
flac_lpc_16_16_c:       17884.0
flac_lpc_16_16_rvv_i32: 12125.7
flac_lpc_16_29_c:       27847.7
flac_lpc_16_29_rvv_i32: 10494.0
flac_lpc_16_32_c:       30051.5
flac_lpc_16_32_rvv_i32: 10355.0