[x265] [arm64] port filterPixelToShort

[Pop, Sebastian]

Thanks again Chen for your careful review and recommendations.

I added the following change to the attached patch as we get better performance:

--- a/source/common/aarch64/ipfilter8.S
+++ b/source/common/aarch64/ipfilter8.S
@@ -35,14 +35,14 @@ function x265_filterPixelToShort_4x4_neon
     movi            v2.8h, #0xe0, lsl #8
     ld1             {v0.s}[0], [x0], x1
     ld1             {v0.s}[1], [x0], x1
-    ld1             {v1.s}[2], [x0], x1
-    ld1             {v1.s}[3], [x0], x1
     ushll           v3.8h, v0.8b, #6
-    ushll2          v4.8h, v1.16b, #6
     add             v3.8h, v3.8h, v2.8h
-    add             v4.8h, v4.8h, v2.8h
     st1             {v3.d}[0], [x2], x3
     st1             {v3.d}[1], [x2], x3
+    ld1             {v1.s}[0], [x0], x1
+    ld1             {v1.s}[1], [x0], x1
+    ushll           v4.8h, v1.8b, #6
+    add             v4.8h, v4.8h, v2.8h
     st1             {v4.d}[0], [x2], x3
     st1             {v4.d}[1], [x2], x3
     ret

Before:
convert_p2s[  4x4]                           1.20x      4.99      6.01

After:
convert_p2s[  4x4]                           1.38x      4.20      5.78

I tried the ldp with post-increment as you recommended.
Performance is slightly lower with the change:

function x265_filterPixelToShort_64x\h\()_neon
     add             x3, x3, x3
     sub             x3, x3, #0x40
+    sub             x1, x1, #0x20
     movi            v4.8h, #0xe0, lsl #8
     mov             x9, #\r
.loop_filterP2S_64x\h:
     subs            x9, x9, #1
.rept 2
-    ld1             {v0.16b-v3.16b}, [x0], x1
+    ldp             q0, q1, [x0], #0x20
+    ldp             q0, q1, [x0]
+    add             x0, x0, x1
     ushll           v16.8h, v0.8b, #6
     ushll2          v17.8h, v0.16b, #6
     ushll           v18.8h, v1.8b, #6

Before:
convert_p2s[64x16]                        1.46x      105.52                  154.47
convert_p2s[64x32]                        1.47x      212.06                  312.14
convert_p2s[64x48]                        1.47x      318.75                  467.61
convert_p2s[64x64]                        1.46x      425.61                  622.36

After:
convert_p2s[64x16]                        1.42x      108.41                  154.37
convert_p2s[64x32]                        1.45x      215.18                  312.12
convert_p2s[64x48]                        1.44x      325.01                  468.76
convert_p2s[64x64]                        1.44x      432.46                  622.36

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