This makes it similar to put_epel16_v6, and gives a 10-25%
speedup of this function.

Before:                   Cortex A7       A8       A9      A53     A72
vp8_put_epel16_h6v6_neon:    3058.0   2218.5   2459.8   2183.0  1572.2
After:
vp8_put_epel16_h6v6_neon:    2670.8   1934.2   2244.4   1729.4  1503.9
Signed-off-by: Martin Storsjö <martin@martin.st>