internal/lz4block: Speed up noasm decoder

[greatroar]

When the compiler is told exactly how many bytes a copy call should copy, and that number is at most 16, it will inline the call. Also, the old code only took the short match shortcut when the short literal shortcut was also taken. But long literals with short matches are common.

Benchmark results on older Intel:

goos: linux
goarch: amd64
pkg: github.com/pierrec/lz4/v4
cpu: Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
                   │     old      │                 new                  │
                   │     B/s      │     B/s       vs base                │
UncompressPg1661-8   327.9Mi ± 1%   549.7Mi ± 0%  +67.61% (p=0.000 n=10)
UncompressDigits-8   1.111Gi ± 1%   1.499Gi ± 1%  +34.94% (p=0.000 n=10)
UncompressTwain-8    348.3Mi ± 0%   579.4Mi ± 0%  +66.32% (p=0.000 n=10)
UncompressRand-8     3.296Gi ± 0%   3.309Gi ± 1%        ~ (p=0.739 n=10)
geomean              813.8Mi        1.108Gi       +39.40%

On newer AMD:

goos: linux
goarch: amd64
pkg: github.com/pierrec/lz4/v4
cpu: AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics
                    │     old      │                  new                  │
                    │     B/s      │      B/s       vs base                │
UncompressPg1661-16   643.6Mi ± 2%   1076.9Mi ± 1%  +67.33% (p=0.000 n=10)
UncompressDigits-16   2.808Gi ± 1%    3.786Gi ± 0%  +34.82% (p=0.000 n=10)
UncompressTwain-16    702.8Mi ± 1%   1309.5Mi ± 7%  +86.32% (p=0.000 n=10)
UncompressRand-16     6.878Gi ± 0%    6.850Gi ± 1%   -0.42% (p=0.009 n=10)
geomean               1.699Gi         2.430Gi       +43.04%

The assembler version still has quite an edge on amd64, so it might be too early to replace it (which would solve #215):

goos: linux
goarch: amd64
pkg: github.com/pierrec/lz4/v4
cpu: Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
                   │     new      │               amd64-v4                │
                   │     B/s      │      B/s       vs base                │
UncompressPg1661-8   549.7Mi ± 0%   1037.5Mi ± 1%  +88.75% (p=0.000 n=10)
UncompressDigits-8   1.499Gi ± 1%    1.954Gi ± 0%  +30.33% (p=0.000 n=10)
UncompressTwain-8    579.4Mi ± 0%   1072.4Mi ± 1%  +85.11% (p=0.000 n=10)
UncompressRand-8     3.309Gi ± 1%    3.306Gi ± 2%        ~ (p=1.000 n=10)
geomean              1.108Gi         1.618Gi       +46.05%

The code passes TestIssue51 without a specific check for it, as well as the unit test from #191, which I haven't included in this PR for copyright reasons.

[greatroar]

The test failure is unrelated to this PR.