Use LDP instructions to load 16 bytes per loop when the source length is long. Specially
process the 8 bytes length, 4 bytes length and 2 bytes length to get a better performance.

Benchmark result:
name                           old time/op   new time/op    delta
BytesCompare/1-8                21.0ns ± 0%    10.5ns ± 0%      ~     (p=0.079 n=4+5)
BytesCompare/2-8                11.5ns ± 0%    10.5ns ± 0%    -8.70%  (p=0.008 n=5+5)
BytesCompare/4-8                13.5ns ± 0%    10.0ns ± 0%   -25.93%  (p=0.008 n=5+5)
BytesCompare/8-8                28.8ns ± 0%     9.5ns ± 0%      ~     (p=0.079 n=4+5)
BytesCompare/16-8               40.5ns ± 0%    10.5ns ± 0%   -74.07%  (p=0.008 n=5+5)
BytesCompare/32-8               64.6ns ± 0%    12.5ns ± 0%   -80.65%  (p=0.008 n=5+5)
BytesCompare/64-8                112ns ± 0%      16ns ± 0%   -85.27%  (p=0.008 n=5+5)
BytesCompare/128-8               208ns ± 0%      24ns ± 0%   -88.22%  (p=0.008 n=5+5)
BytesCompare/256-8               400ns ± 0%      50ns ± 0%   -87.62%  (p=0.008 n=5+5)
BytesCompare/512-8               785ns ± 0%      82ns ± 0%   -89.61%  (p=0.008 n=5+5)
BytesCompare/1024-8             1.55µs ± 0%    0.14µs ± 0%      ~     (p=0.079 n=4+5)
BytesCompare/2048-8             3.09µs ± 0%    0.27µs ± 0%      ~     (p=0.079 n=4+5)
CompareBytesEqual-8             39.0ns ± 0%    12.0ns ± 0%   -69.23%  (p=0.008 n=5+5)
CompareBytesToNil-8             8.57ns ± 5%    8.23ns ± 2%    -3.99%  (p=0.016 n=5+5)
CompareBytesEmpty-8             7.37ns ± 0%    7.36ns ± 4%      ~     (p=0.690 n=5+5)
CompareBytesIdentical-8         7.39ns ± 0%    7.46ns ± 2%      ~     (p=0.667 n=5+5)
CompareBytesSameLength-8        17.0ns ± 0%    10.5ns ± 0%   -38.24%  (p=0.008 n=5+5)
CompareBytesDifferentLength-8   17.0ns ± 0%    10.5ns ± 0%   -38.24%  (p=0.008 n=5+5)
CompareBytesBigUnaligned-8      1.58ms ± 0%    0.19ms ± 0%   -88.31%  (p=0.016 n=4+5)
CompareBytesBig-8               1.59ms ± 0%    0.19ms ± 0%   -88.27%  (p=0.016 n=5+4)
CompareBytesBigIdentical-8      7.01ns ± 0%    6.60ns ± 3%    -5.91%  (p=0.008 n=5+5)

name                           old speed     new speed      delta
CompareBytesBigUnaligned-8     662MB/s ± 0%  5660MB/s ± 0%  +755.15%  (p=0.016 n=4+5)
CompareBytesBig-8              661MB/s ± 0%  5636MB/s ± 0%  +752.57%  (p=0.016 n=5+4)
CompareBytesBigIdentical-8     150TB/s ± 0%   159TB/s ± 3%    +6.27%  (p=0.008 n=5+5)

This is resubmit of CL90175.

Change-Id: Ie841daedb3123a68dd2554f27ebef0b3f8a855c2
Reviewed-on: https://go-review.googlesource.com/101635
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>