Summing 16 bit hamming codes restricts the maximum length,
but saves an inner loop instruction.  The outer loop can sum the
values.

32 bit Neon
Now BenchmarkHammingDistance_Opt (78 ms)
Was BenchmarkHammingDistance_Opt (92 ms)

64 bit Neon
Now BenchmarkHammingDistance_Opt (85 ms)
Was BenchmarkHammingDistance_Opt (92 ms)

R=wangcheng@google.com
TBR=kjellander@chromium.org
BUG=libyuv:701
TEST=BenchmarkHammingDistance

Change-Id: Ie40f0eac2f3339c33b833b42af5d394b122066ae
Reviewed-on: https://chromium-review.googlesource.com/526932Reviewed-by: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>