arm64: itx16: Use usqadd to avoid separate clamping of negative values

Before:                                Cortex A53     A72      A73
inv_txfm_add_4x4_dct_dct_0_10bpc_neon:       40.7    23.0     24.0
inv_txfm_add_4x4_dct_dct_1_10bpc_neon:      116.0    71.5     78.2
inv_txfm_add_8x8_dct_dct_0_10bpc_neon:       85.7    50.7     53.8
inv_txfm_add_8x8_dct_dct_1_10bpc_neon:      287.0   203.5    215.2
inv_txfm_add_16x16_dct_dct_0_10bpc_neon:    255.7   129.1    140.4
inv_txfm_add_16x16_dct_dct_1_10bpc_neon:   1401.4  1026.7   1039.2
inv_txfm_add_16x16_dct_dct_2_10bpc_neon:   1913.2  1407.3   1479.6
After:
inv_txfm_add_4x4_dct_dct_0_10bpc_neon:       38.7    21.5     22.2
inv_txfm_add_4x4_dct_dct_1_10bpc_neon:      116.0    71.3     77.2
inv_txfm_add_8x8_dct_dct_0_10bpc_neon:       76.7    44.7     43.5
inv_txfm_add_8x8_dct_dct_1_10bpc_neon:      278.0   203.0    203.9
inv_txfm_add_16x16_dct_dct_0_10bpc_neon:    236.9   106.2    116.2
inv_txfm_add_16x16_dct_dct_1_10bpc_neon:   1368.7   999.7   1008.4
inv_txfm_add_16x16_dct_dct_2_10bpc_neon:   1880.5  1381.2   1459.4
35 jobs for arm64-usq in 5 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #547401
amd64 docker
style-check

00:00:06

passed #547402
amd64 docker
x86inc-check

00:00:06

 
  Build
passed #547414
amd64 docker
build-android-aarch64

00:00:10

passed #547413
amd64 docker
build-android-armv7

00:00:11

passed #547403
amd64 avx2 docker
build-debian

00:00:24

passed #547415
aarch64 docker
build-debian-aarch64

00:00:17

passed #547416
aarch64 docker
build-debian-aarch64-clang-5

00:00:14

passed #547419
armv7 docker
build-debian-armv7

00:00:45

passed #547420
armv7 docker
build-debian-armv7-clang-5

00:00:34

passed #547406
amd64 docker
build-debian-examples

00:00:18

passed #547407
amd64 docker
build-debian-no-tools

00:00:22

passed #547422
ppc64le docker
build-debian-ppc64le

00:01:43

passed #547404
amd64 docker
build-debian-static

00:00:22

passed #547418
aarch64 docker
build-debian-werror

00:00:10

passed #547405
amd64 docker
build-debian32

00:00:25

passed #547417
catalina amd64
build-macos

00:00:41

passed #547423
amd64 docker
build-pages

00:00:09

passed #547421
amd64 docker
build-ubuntu-snap

00:00:26

passed #547411
amd64 docker
build-win-arm32

00:00:11

passed #547412
amd64 docker
build-win-arm64

00:00:14

passed #547408
amd64 docker
build-win32

00:00:39

passed #547409
amd64 docker
build-win32-unaligned-stack

00:00:32

passed #547410
amd64 docker
build-win64

00:00:39

 
  Test
passed #547424
amd64 docker
test-debian

00:01:10

95.8258%
passed #547433
aarch64 docker
test-debian-aarch64

00:00:26

passed #547435
armv7 docker
test-debian-armv7-clang-5

00:01:09

passed #547428
amd64 docker
test-debian-asan

00:01:56

passed #547425
avx2 amd64 docker
test-debian-asm

00:01:03

passed #547429
amd64 docker
test-debian-msan

00:01:11

passed #547434
ppc64le docker
test-debian-ppc64le

00:02:49

passed #547431
amd64 docker
test-debian-tsan

00:03:38

passed #547430
amd64 docker
test-debian-ubsan

00:01:59

passed #547427
amd64 avx2 docker
test-debian-unaligned-stack

00:00:49

passed #547426
avx2 amd64 docker
test-debian32-asm

00:01:00

passed #547432
amd64 avx2 docker
test-win64

00:01:09