lavc/aarch64: Fix addp overflow in ff_pred16x16_plane_neon_10

The mismatch between neon and C functions can be reproduced
using the following bitstream and command line on aarch64 devices.

wget https://streams.videolan.org/ffmpeg/incoming/replay_intra_pred_16x16.h264
 ./ffmpeg -cpuflags 0  -threads 1 -i replay_intra_pred_16x16.h264  -f framemd5 -y md5_ref
 ./ffmpeg              -threads 1 -i replay_intra_pred_16x16.h264 -f framemd5 -y md5_neon

Signed-off-by: Bin Peng <pengbin@visionular.com>

configure

@@ -3126,7 +3126,7 @@ mts2_decoder_select="jpegtables mss34dsp"
 mv30_decoder_select="aandcttables blockdsp"
 mvha_decoder_select="inflate_wrapper llviddsp"
 mwsc_decoder_select="inflate_wrapper"
-mxpeg_decoder_select="mjpeg_decoder"
+mxpeg_decoder_select="hpeldsp mjpeg_decoder"
 nellymoser_decoder_select="sinewin"
 nellymoser_encoder_select="audio_frame_queue sinewin"
 notchlc_decoder_select="lzf"

libavcodec/aarch64/h264pred_neon.S

@@ -489,10 +489,10 @@ function ff_pred16x16_plane_neon_10, export=1
         mul             v2.8h,  v2.8h,  v0.8h
         mul             v3.8h,  v3.8h,  v0.8h
         addp            v2.8h,  v2.8h,  v3.8h
-        addp            v2.8h,  v2.8h,  v2.8h
-        addp            v2.4h,  v2.4h,  v2.4h
-        sshll           v3.4s,  v2.4h,  #2
-        saddw           v2.4s,  v3.4s,  v2.4h
+        saddlp          v2.4s,  v2.8h
+        addp            v2.4s,  v2.4s,  v2.4s
+        shl             v3.4s,  v2.4s,  #2
+        add             v2.4s,  v3.4s,  v2.4s
         rshrn           v4.4h,  v2.4s,  #6
         trn2            v5.4h,  v4.4h,  v4.4h
         add             v2.4h,  v4.4h,  v5.4h
@@ -506,14 +506,13 @@ function ff_pred16x16_plane_neon_10, export=1
         sxtl            v6.4s,  v5.4h          // c
 
         mov             v0.h[0],  wzr
-        mul             v0.8h,  v0.8h,  v4.h[0]
         dup             v16.4s, v2.s[0]
         dup             v17.4s, v2.s[0]
         dup             v2.8h,  v4.h[0]        // b
         dup             v3.4s,  v6.s[0]        // c
         sshll           v2.4s,  v2.4h,  #3     // b * 8
-        saddw           v16.4s, v16.4s, v0.4h
-        saddw2          v17.4s, v17.4s, v0.8h
+        smlal           v16.4s, v0.4h, v4.h[0]
+        smlal2          v17.4s, v0.8h, v4.h[0]
         sub             v3.4s,  v3.4s,  v2.4s
 
         mov             w3,      #16