[x265] [arm64] port filterPixelToShort

[Pop, Sebastian]

I added the following change in the attached patch.
It has better performance with ldp as it allows to re-schedule the instructions in independent ways:

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
     ushll           v16.8h, v0.8b, #6
     ushll2          v17.8h, v0.16b, #6
     ushll           v18.8h, v1.8b, #6
     ushll2          v19.8h, v1.16b, #6
-    ushll           v20.8h, v2.8b, #6
-    ushll2          v21.8h, v2.16b, #6
-    ushll           v22.8h, v3.8b, #6
-    ushll2          v23.8h, v3.16b, #6
     add             v16.8h, v16.8h, v4.8h
     add             v17.8h, v17.8h, v4.8h
     add             v18.8h, v18.8h, v4.8h
     add             v19.8h, v19.8h, v4.8h
+    st1             {v16.16b-v19.16b}, [x2], #0x40
+
+    ldp             q2, q3, [x0]
+    add             x0, x0, x1
+    ushll           v20.8h, v2.8b, #6
+    ushll2          v21.8h, v2.16b, #6
+    ushll           v22.8h, v3.8b, #6
+    ushll2          v23.8h, v3.16b, #6
     add             v20.8h, v20.8h, v4.8h
     add             v21.8h, v21.8h, v4.8h
     add             v22.8h, v22.8h, v4.8h
     add             v23.8h, v23.8h, v4.8h
-    st1             {v16.16b-v19.16b}, [x2], #0x40
     st1             {v20.16b-v23.16b}, [x2], x3
.endr
     bgt             .loop_filterP2S_64x\h

Before:
convert_p2s[64x16]                        1.46x      105.51                  154.37
convert_p2s[64x32]                        1.47x      212.07                  312.12
convert_p2s[64x48]                        1.46x      318.76                  466.80
convert_p2s[64x64]                        1.47x      425.34                  623.56

After:
convert_p2s[64x16]                        1.47x      105.24                  154.46
convert_p2s[64x32]                        1.50x      207.42                  312.09
convert_p2s[64x48]                        1.49x      312.30                  466.27
convert_p2s[64x64]                        1.50x      415.77                  623.56
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