Add SSSE3 implementations for dav1d_ipred_top, dav1d_ipred_left and dav1d_ipred_128

Cycle times:
intra_pred_dc_128_w4_8bpc_c: 905.2
intra_pred_dc_128_w4_8bpc_ssse3: 61.6
intra_pred_dc_128_w8_8bpc_c: 1393.1
intra_pred_dc_128_w8_8bpc_ssse3: 82.3
intra_pred_dc_128_w16_8bpc_c: 2227.4
intra_pred_dc_128_w16_8bpc_ssse3: 119.6
intra_pred_dc_128_w32_8bpc_c: 2696.0
intra_pred_dc_128_w32_8bpc_ssse3: 195.5
intra_pred_dc_128_w64_8bpc_c: 4298.6
intra_pred_dc_128_w64_8bpc_ssse3: 465.1
intra_pred_dc_left_w4_8bpc_c: 974.2
intra_pred_dc_left_w4_8bpc_ssse3: 80.2
intra_pred_dc_left_w8_8bpc_c: 1478.4
intra_pred_dc_left_w8_8bpc_ssse3: 103.7
intra_pred_dc_left_w16_8bpc_c: 2313.0
intra_pred_dc_left_w16_8bpc_ssse3: 159.1
intra_pred_dc_left_w32_8bpc_c: 2835.1
intra_pred_dc_left_w32_8bpc_ssse3: 305.3
intra_pred_dc_left_w64_8bpc_c: 4462.2
intra_pred_dc_left_w64_8bpc_ssse3: 525.5
intra_pred_dc_top_w4_8bpc_c: 949.5
intra_pred_dc_top_w4_8bpc_ssse3: 95.5
intra_pred_dc_top_w8_8bpc_c: 1462.2
intra_pred_dc_top_w8_8bpc_ssse3: 103.1
intra_pred_dc_top_w16_8bpc_c: 2312.5
intra_pred_dc_top_w16_8bpc_ssse3: 146.4
intra_pred_dc_top_w32_8bpc_c: 2895.9
intra_pred_dc_top_w32_8bpc_ssse3: 250.4
intra_pred_dc_top_w64_8bpc_c: 4617.9
intra_pred_dc_top_w64_8bpc_ssse3: 493.3
14 jobs for master in 5 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #245952
amd64 debian
style-check

00:00:20

 
  Build
passed #245953
amd64 debian
build-debian

00:00:33

passed #245958
debian aarch64
build-debian-aarch64

00:01:34

passed #245959
debian aarch64
build-debian-aarch64-clang-5

00:01:03

passed #245954
amd64 debian
build-debian-static

00:00:35

passed #245961
debian aarch64
build-debian-werror

00:00:33

passed #245955
amd64 debian
build-debian32

00:00:25

passed #245960
macos
build-macos

00:00:27

passed #245956
win32
build-win32

00:00:31

passed #245957
win64
build-win64

00:00:35

 
  Test
passed #245962
amd64 debian
test-debian

00:00:44

passed #245963
amd64 debian
test-debian-asan

00:02:04

passed #245964
amd64 debian
test-debian-msan

00:00:58

passed #245965
amd64 debian
test-debian-ubsan

00:01:18