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arm64: loopfilter16: Fix conditions for skipping parts of the filtering

As the arm64 16 bpc loopfilter operates on a 8 pixel region at a time,
inspect 2 bits (corresponding to 4 pixels each) from these registers,
as we also shift them down by 2 bits at the end of the loop.

This should allow skipping the loopfilter altogether (or using a
smaller filter) in more cases.

--- a/src/arm/64/loopfilter16.S

+++ b/src/arm/64/loopfilter16.S

@@ -785,7 +785,7 @@

         orr             w6,  w6,  w7             // vmask[0] |= vmask[1]

 1:

-        tst             w6,  #0x0f

+        tst             w6,  #0x03

 .ifc \dir, v

         ld1             {v0.8b}, [x4], #8

         ld1             {v1.8b}, [x3], #8

@@ -847,7 +847,7 @@

         ushl            v10.8h,  v10.8h,  v31.8h

 .ifc \type, y

-        tst             w2,  #0x0f

+        tst             w2,  #0x03

         b.eq            2f

         // wd16

         bl              lpf_\dir\()_16_8_neon

@@ -854,7 +854,7 @@

         b               8f

 2:

 .endif

-        tst             w7,  #0x0f

+        tst             w7,  #0x03

         b.eq            3f

 .ifc \type, y

         // wd8