Roll the loop to avoid slow gathers.

Before:
vector_fmul_reverse_c:       1561.7
vector_fmul_reverse_rvv_f32: 2410.2
vector_fmul_window_c:        2068.2
vector_fmul_window_rvv_f32:  1879.5

After:
vector_fmul_reverse_c:       1561.7
vector_fmul_reverse_rvv_f32:  916.2
vector_fmul_window_c:        2068.2
vector_fmul_window_rvv_f32:  1202.5