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arm64: itx: Minor optimizations for the 8x32 functions

This gives a couple cycles speedup.

--- a/src/arm/64/itx.S

+++ b/src/arm/64/itx.S

@@ -2317,6 +2317,7 @@

         mov             w8,  #2*\h

 1:

+        ldrh            w12, [x13], #2

 .irp i, v16.8h, v17.8h, v18.8h, v19.8h, v20.8h, v21.8h, v22.8h, v23.8h

         ld1             {\i}, [x2]

         st1             {v0.8h}, [x2], x8

@@ -2329,6 +2330,7 @@

         transpose_8x8h  v16, v17, v18, v19, v20, v21, v22, v23, v4, v5

+        cmp             w3,  w12

 .if \w == 8

         load_add_store_8x8 x0, x7, shiftbits=2

 .else

@@ -2335,8 +2337,6 @@

         load_add_store_8x8 x0, x7, shiftbits=3

 .endif

-        ldrh            w12, [x13], #2

-        cmp             w3,  w12

         b.lt            9f

 .if \w == 8

         sub             x2,  x2,  x8, lsl #3

@@ -2509,16 +2509,15 @@

         mov             x8,  #2*32

         mov             w9,  #32

         mov             x6,  sp

-        mov             x7,  x2

 1:

 .irp i, 16, 17, 18, 19, 20, 21, 22, 23

-        ld1             {v\i\().8h}, [x7]

-        st1             {v28.8h}, [x7], x8

+        ld1             {v\i\().8h}, [x2]

+        st1             {v28.8h}, [x2], x8

 .endr

         ldrh            w12, [x13], #2

+        sub             x2,  x2,  x8, lsl #3

         sub             w9,  w9,  #8

-        sub             x7,  x7,  x8, lsl #3

-        add             x7,  x7,  #2*8

+        add             x2,  x2,  #2*8

         bl              inv_dct_8x8_neon

@@ -2528,10 +2527,9 @@

         transpose_8x8h  v16, v17, v18, v19, v20, v21, v22, v23, v24, v25

+        st1             {v16.8h, v17.8h, v18.8h, v19.8h}, [x6], #64

         cmp             w3,  w12

-.irp i, 16, 17, 18, 19, 20, 21, 22, 23

-        st1             {v\i\().8h}, [x6], #16

-.endr

+        st1             {v20.8h, v21.8h, v22.8h, v23.8h}, [x6], #64

         b.ge            1b

         cbz             w9,  3f