bytes,strings: use IndexByte more often in Index on AMD64

bytes,strings: use IndexByte more often in Index on AMD64

IndexByte+compare is faster than indexShortStr in good case, when
first byte is rare, but is more costly in bad cases.
Start with IndexByte and switch to indexShortStr if we encounter
false positives more often than once per 8 bytes.

Benchmark changes for package bytes:

IndexRune/4K-8                    416ns ± 0%       86ns ± 0%    -79.24%        (p=0.000 n=10+10)
IndexRune/4M-8                    413µs ± 0%      100µs ± 1%    -75.88%        (p=0.000 n=10+10)
IndexRune/64M-8                  6.73ms ± 0%     2.86ms ± 1%    -57.49%        (p=0.000 n=10+10)
Index/10-8                       8.45ns ± 0%     8.96ns ± 0%     +6.04%         (p=0.000 n=9+10)
Index/32-8                       9.64ns ± 0%     9.51ns ± 0%     -1.30%          (p=0.000 n=8+9)
Index/4K-8                       2.11µs ± 0%     2.12µs ± 0%     +0.26%        (p=0.000 n=10+10)
Index/4M-8                       3.60ms ± 5%     3.59ms ± 7%       ~            (p=0.497 n=9+10)
Index/64M-8                      57.1ms ± 3%     58.7ms ± 5%       ~            (p=0.113 n=9+10)
IndexEasy/10-8                   7.10ns ± 1%     7.71ns ± 1%     +8.60%        (p=0.000 n=10+10)
IndexEasy/32-8                   9.29ns ± 1%     9.22ns ± 0%     -0.75%         (p=0.000 n=9+10)
IndexEasy/4K-8                   1.06µs ± 0%     0.08µs ± 0%    -92.18%        (p=0.000 n=10+10)
IndexEasy/4M-8                   1.07ms ± 0%     0.10ms ± 1%    -90.74%         (p=0.000 n=9+10)
IndexEasy/64M-8                  17.3ms ± 0%      2.8ms ± 1%    -83.76%         (p=0.000 n=10+9)

IndexRune/4K-8                 9.84GB/s ± 0%  47.42GB/s ± 0%   +381.85%         (p=0.000 n=8+10)
IndexRune/4M-8                 10.1GB/s ± 0%   42.1GB/s ± 1%   +314.56%        (p=0.000 n=10+10)
IndexRune/64M-8                10.0GB/s ± 0%   23.4GB/s ± 1%   +135.25%        (p=0.000 n=10+10)
Index/10-8                     1.18GB/s ± 0%   1.12GB/s ± 0%     -5.67%         (p=0.000 n=10+9)
Index/32-8                     3.32GB/s ± 0%   3.36GB/s ± 0%     +1.27%         (p=0.000 n=10+9)
Index/4K-8                     1.94GB/s ± 0%   1.93GB/s ± 0%     -0.25%         (p=0.000 n=10+9)
Index/4M-8                     1.17GB/s ± 5%   1.17GB/s ± 7%       ~            (p=0.497 n=9+10)
Index/64M-8                    1.17GB/s ± 3%   1.15GB/s ± 6%       ~            (p=0.113 n=9+10)
IndexEasy/10-8                 1.41GB/s ± 1%   1.30GB/s ± 1%     -7.90%        (p=0.000 n=10+10)
IndexEasy/32-8                 3.45GB/s ± 1%   3.47GB/s ± 0%     +0.73%         (p=0.000 n=9+10)
IndexEasy/4K-8                 3.84GB/s ± 0%  49.16GB/s ± 0%  +1178.78%         (p=0.000 n=9+10)
IndexEasy/4M-8                 3.91GB/s ± 0%  42.19GB/s ± 1%   +980.37%         (p=0.000 n=9+10)
IndexEasy/64M-8                3.88GB/s ± 0%  23.91GB/s ± 1%   +515.76%         (p=0.000 n=10+9)

No significant changes in strings.

In regexp I see:

Match/Easy0/32-8                 536MB/s ± 1%   540MB/s ± 1%    +0.75%         (p=0.001 n=9+10)
Match/Easy0/1K-8                1.62GB/s ± 0%  4.42GB/s ± 1%  +172.48%        (p=0.000 n=10+10)
Match/Easy0/32K-8               1.87GB/s ± 0%  9.07GB/s ± 1%  +384.24%         (p=0.000 n=7+10)
Match/Easy0/1M-8                1.90GB/s ± 0%  4.83GB/s ± 0%  +154.56%         (p=0.000 n=8+10)
Match/Easy0/32M-8               1.90GB/s ± 0%  4.53GB/s ± 0%  +138.62%         (p=0.000 n=7+10)

Compared to in 1.7:

Match/Easy0/32-8                  59.5ns ± 0%    59.2ns ± 1%   -0.45%         (p=0.008 n=9+10)
Match/Easy0/1K-8                   226ns ± 1%     231ns ± 1%   +2.30%        (p=0.000 n=10+10)
Match/Easy0/32K-8                 3.73µs ± 2%    3.61µs ± 1%   -3.12%        (p=0.000 n=10+10)
Match/Easy0/1M-8                   206µs ± 1%     217µs ± 0%   +5.34%        (p=0.000 n=10+10)
Match/Easy0/32M-8                 7.03ms ± 1%    7.40ms ± 0%   +5.23%        (p=0.000 n=10+10)

Fixes #17456

Change-Id: I38b2fabcaed7119cc4bf37007ba7bfe7504c8f9f
Reviewed-on: https://go-review.googlesource.com/31690
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>