This makes it similar to put_epel16_v6, and gives a large speedup
on Cortex A53, a minor speedup on A72 and a very minor slowdown on
A73.

Before:                 Cortex A53     A72     A73
vp8_put_epel16_h6v6_neon:   2211.4  1586.5  1431.7
After:
vp8_put_epel16_h6v6_neon:   1736.9  1522.0  1448.1
Signed-off-by: Martin Storsjö <martin@martin.st>