Also use aligned reads and writes in sub_loop, and integrate sum_loop into
the main loop.

before:
cfl_ac_420_w4_8bpc_c: 367.4
cfl_ac_420_w4_8bpc_avx2: 72.8
cfl_ac_420_w8_8bpc_c: 621.6
cfl_ac_420_w8_8bpc_avx2: 85.1
cfl_ac_420_w16_8bpc_c: 983.4
cfl_ac_420_w16_8bpc_avx2: 141.0

after:
cfl_ac_420_w4_8bpc_c: 376.2
cfl_ac_420_w4_8bpc_avx2: 28.5
cfl_ac_420_w8_8bpc_c: 607.2
cfl_ac_420_w8_8bpc_avx2: 29.9
cfl_ac_420_w16_8bpc_c: 962.1
cfl_ac_420_w16_8bpc_avx2: 48.8