Except for add_squares, telling the compiler that the output vector(s)
cannot alias helps quite a bit (cycles on SiFive U74-MC):

ps_add_squares_c: 98277.7
ps_add_squares_r: 98320.2

ps_hybrid_analysis_c: 3731.2
ps_hybrid_analysis_r: 2495.7

ps_hybrid_analysis_ileave_c: 20478.0
ps_hybrid_analysis_ileave_r: 16092.2

ps_hybrid_synthesis_deint_c: 19051.5
ps_hybrid_synthesis_deint_r: 15420.0

ps_mul_pair_single_c: 122941.2
ps_mul_pair_single_r: 91035.0