Add SSSE3 implementation for dav1d_ipred_v and dav1d_ipred_dc

Cycle times:
intra_pred_dc_w4_8bpc_c: 1051.4
intra_pred_dc_w4_8bpc_ssse3: 58.8
intra_pred_dc_w8_8bpc_c: 1587.6
intra_pred_dc_w8_8bpc_ssse3: 75.3
intra_pred_dc_w16_8bpc_c: 2526.2
intra_pred_dc_w16_8bpc_ssse3: 103.5
intra_pred_dc_w32_8bpc_c: 2646.6
intra_pred_dc_w32_8bpc_ssse3: 179.5
intra_pred_dc_w64_8bpc_c: 4084.6
intra_pred_dc_w64_8bpc_ssse3: 356.1
intra_pred_v_w4_8bpc_c: 468.5
intra_pred_v_w4_8bpc_ssse3: 46.8
intra_pred_v_w8_8bpc_c: 839.1
intra_pred_v_w8_8bpc_ssse3: 56.7
intra_pred_v_w16_8bpc_c: 1750.5
intra_pred_v_w16_8bpc_ssse3: 73.0
intra_pred_v_w32_8bpc_c: 1552.5
intra_pred_v_w32_8bpc_ssse3: 135.4
intra_pred_v_w64_8bpc_c: 2463.6
intra_pred_v_w64_8bpc_ssse3: 305.6
14 jobs for master in 5 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style
passed #245651
amd64 debian
style-check

00:00:21

 
  Build
passed #245652
amd64 debian
build-debian

00:00:32

passed #245657
debian aarch64
build-debian-aarch64

00:01:30

passed #245658
debian aarch64
build-debian-aarch64-clang-5

00:01:06

passed #245653
amd64 debian
build-debian-static

00:00:35

passed #245660
debian aarch64
build-debian-werror

00:00:34

passed #245654
amd64 debian
build-debian32

00:00:25

passed #245659
macos
build-macos

00:00:27

passed #245655
win32
build-win32

00:00:31

passed #245656
win64
build-win64

00:00:36

 
  Test
passed #245661
amd64 debian
test-debian

00:00:48

passed #245662
amd64 debian
test-debian-asan

00:02:02

passed #245663
amd64 debian
test-debian-msan

00:01:03

passed #245664
amd64 debian
test-debian-ubsan

00:01:22