arm: vp9mc: Load only 12 pixels in the 4 pixel wide horizontal filter

This reduces the amount the horizontal filters read beyond the filter
width to a consistent 1 pixel. The data is not used so this is usually
not noticeable. It becomes a problem when the application allocates
frame buffers only for the aligned picture size and the end of it is at
a page boundary. This happens for picture sizes which are a multiple of
the page size like 1280x640. The frame buffer allocation is based on
its most likely done via mmap + MAP_ANONYMOUS so start and end of the
buffer are page aligned and the previous and next page are not
necessarily mapped.
This mirrors the aarch64 change.

Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit f366256215)

libavcodec/arm/vp9mc_neon.S

@@ -279,11 +279,13 @@ function \type\()_8tap_\size\()h_\idx1\idx2
         sub             r1,  r1,  r5
 .endif
         @ size >= 16 loads two qwords and increments r2,
-        @ for size 4/8 it's enough with one qword and no
-        @ postincrement
+        @ size 4 loads 1 d word, increments r2 and loads 1 32-bit lane
+        @ for size 8 it's enough with one qword and no postincrement
 .if \size >= 16
         sub             r3,  r3,  r5
         sub             r3,  r3,  #8
+.elseif \size == 4
+        sub             r3,  r3,  #8
 .endif
         @ Load the filter vector
         vld1.16         {q0},  [r12,:128]
@@ -295,9 +297,14 @@ function \type\()_8tap_\size\()h_\idx1\idx2
 .if \size >= 16
         vld1.8          {d18, d19, d20}, [r2]!
         vld1.8          {d24, d25, d26}, [r7]!
-.else
+.elseif \size == 8
         vld1.8          {q9},  [r2]
         vld1.8          {q12}, [r7]
+.else @ size == 4
+        vld1.8          {d18}, [r2]!
+        vld1.8          {d24}, [r7]!
+        vld1.32         {d19[0]}, [r2]
+        vld1.32         {d25[0]}, [r7]
 .endif
         vmovl.u8        q8,  d18
         vmovl.u8        q9,  d19