arm32: ipred: NEON implementation of ipred functions for 16 bpc

Samples of some checkasm benchmarks:
                                 Cortex A7       A8      A53      A72      A73
cfl_ac_420_w4_16bpc_neon:            258.2    130.0    187.8     88.1     99.9
cfl_ac_420_w8_16bpc_neon:            396.3    192.3    278.0    134.1    148.1
cfl_ac_420_w16_16bpc_neon:           705.9    341.5    508.4    231.2    263.0
intra_pred_filter_w32_10bpc_neon:   3450.6   3279.7   1505.6   1716.8   1631.0
intra_pred_filter_w32_12bpc_neon:   5075.2   2467.3   2027.9   1605.7   1556.0
intra_pred_paeth_w64_16bpc_neon:    7850.6   4682.9   4538.4   4640.4   4952.4
intra_pred_smooth_w64_16bpc_neon:   6807.7   4044.0   4001.4   3001.9   3131.5

Corresponding numbers for arm64:                  Cortex A53      A72      A73
cfl_ac_420_w4_16bpc_neon:                              154.8     87.1     81.6
cfl_ac_420_w8_16bpc_neon:                              235.6    124.8    133.0
cfl_ac_420_w16_16bpc_neon:                             428.8    206.5    234.9
intra_pred_filter_w32_10bpc_neon:                     1333.2   1485.9   1468.3
intra_pred_filter_w32_12bpc_neon:                     1839.1   1429.0   1439.7
intra_pred_paeth_w64_16bpc_neon:                      3691.1   3091.8   3289.7
intra_pred_smooth_w64_16bpc_neon:                     3776.8   3124.4   2827.1
34 jobs for arm32-ipred16 in 8 minutes and 34 seconds (queued for 1 minute and 55 seconds)
Status Job ID Name Coverage
  Style
passed #517665
amd64 docker
style-check

00:00:13

passed #517666
amd64 docker
x86inc-check

00:00:13

 
  Build
passed #517677
amd64 docker
build-android-aarch64

00:00:10

passed #517676
amd64 docker
build-android-armv7

00:00:11

passed #517667
amd64 avx2 docker
build-debian

00:00:41

passed #517678
aarch64 docker
build-debian-aarch64

00:00:25

passed #517679
aarch64 docker
build-debian-aarch64-clang-5

00:00:24

passed #517682
armv7 docker
build-debian-armv7

00:00:45

passed #517683
armv7 docker
build-debian-armv7-clang-5

00:00:33

passed #517670
amd64 docker
build-debian-examples

00:00:15

passed #517685
ppc64le docker
build-debian-ppc64le

00:00:58

passed #517668
amd64 docker
build-debian-static

00:00:22

passed #517681
aarch64 docker
build-debian-werror

00:00:21

passed #517669
amd64 docker
build-debian32

00:00:31

passed #517680
macos
build-macos

00:01:24

passed #517686
amd64 docker
build-pages

00:00:08

passed #517684
amd64 docker
build-ubuntu-snap

00:00:20

passed #517674
amd64 docker
build-win-arm32

00:00:11

passed #517675
amd64 docker
build-win-arm64

00:00:12

passed #517671
amd64 docker
build-win32

00:00:30

passed #517672
amd64 docker
build-win32-unaligned-stack

00:00:24

passed #517673
amd64 docker
build-win64

00:00:41

 
  Test
passed #517687
amd64 docker
test-debian

00:00:46

97.8712%
passed #517696
aarch64 docker
test-debian-aarch64

00:00:40

passed #517698
armv7 docker
test-debian-armv7-clang-5

00:00:53

passed #517691
amd64 docker
test-debian-asan

00:01:32

passed #517688
avx2 amd64 docker
test-debian-asm

00:00:57

passed #517692
amd64 docker
test-debian-msan

00:00:40

passed #517697
ppc64le docker
test-debian-ppc64le

00:01:28

passed #517694
amd64 docker
test-debian-tsan

00:01:12

passed #517693
amd64 docker
test-debian-ubsan

00:01:01

passed #517690
amd64 avx2 docker
test-debian-unaligned-stack

00:00:30

passed #517689
avx2 amd64 docker
test-debian32-asm

00:01:00

passed #517695
amd64 avx2 docker
test-win64

00:01:00