Currently the 16-byte loop chunk16_loop is implemented with NEON instructions LD1, VMOV and VCMEQ.
Using scalar instructions LDP and CMP to achieve this loop can reduce the number of clock cycles.
For cases where the length of strings are between 4 to 15 bytes, loading the last 8 or 4 bytes at
a time to reduce the number of comparisons.

Benchmarks:
name                 old time/op    new time/op    delta
Equal/0-8              5.51ns ± 0%    5.84ns ±14%     ~     (p=0.246 n=7+8)
Equal/1-8              10.5ns ± 0%    10.5ns ± 0%     ~     (all equal)
Equal/6-8              14.0ns ± 0%    12.5ns ± 0%  -10.71%  (p=0.000 n=8+8)
Equal/9-8              13.5ns ± 0%    12.5ns ± 0%   -7.41%  (p=0.000 n=8+8)
Equal/15-8             15.5ns ± 0%    12.5ns ± 0%  -19.35%  (p=0.000 n=8+8)
Equal/16-8             14.0ns ± 0%    13.0ns ± 0%   -7.14%  (p=0.000 n=8+8)
Equal/20-8             16.5ns ± 0%    16.0ns ± 0%   -3.03%  (p=0.000 n=8+8)
Equal/32-8             16.5ns ± 0%    15.3ns ± 0%   -7.27%  (p=0.000 n=8+8)
Equal/4K-8              552ns ± 0%     553ns ± 0%     ~     (p=0.315 n=8+8)
Equal/4M-8             1.13ms ±23%    1.20ms ±27%     ~     (p=0.442 n=8+8)
Equal/64M-8            32.9ms ± 0%    32.6ms ± 0%   -1.15%  (p=0.000 n=8+8)
CompareBytesEqual-8    12.0ns ± 0%    12.0ns ± 0%     ~     (all equal)

Change-Id: If317ecdcc98e31883d37fd7d42b113b548c5bd2a
Reviewed-on: https://go-review.googlesource.com/112496Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>