unicode/utf8: AppendRune and EncodeRune performance improvement

unicode/utf8: AppendRune and EncodeRune performance improvement

- Prefer the evaluation of the valid higher byte-width runes branches
over the one for invalid ones
- Avoid the evaluation of the bytes of the RuneError constant, and
instead hard code its byte values
- EncodeRune only: inline for fast handling of ASCII

goos: linux
goarch: amd64
pkg: unicode/utf8
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
                              │ baseline.test-append.txt │ append-rune-invalid-case-last.test.txt │
                              │          sec/op          │     sec/op      vs base                │
AppendASCIIRune-8                           0.2135n ± 0%     0.2135n ± 0%        ~ (p=0.578 n=20)
AppendSpanishRune-8                          1.645n ± 1%      1.509n ± 2%   -8.30% (p=0.000 n=20)
AppendJapaneseRune-8                         2.196n ± 1%      2.004n ± 1%   -8.74% (p=0.000 n=20)
AppendMaxRune-8                              2.670n ± 1%      2.349n ± 3%  -12.01% (p=0.000 n=20)
AppendInvalidRuneMaxPlusOne-8                2.214n ± 2%      1.798n ± 3%  -18.77% (p=0.000 n=20)
AppendInvalidRuneSurrogate-8                 2.258n ± 1%      1.793n ± 2%  -20.59% (p=0.000 n=20)
AppendInvalidRuneNegative-8                  2.171n ± 2%      1.767n ± 2%  -18.61% (p=0.000 n=20)
geomean                                      1.559n           1.361n       -12.69%

goos: linux
goarch: amd64
pkg: unicode/utf8
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
                              │ baseline.test-encode.txt │     encode-rune-invalid-last.txt     │
                              │          sec/op          │    sec/op     vs base                │
EncodeASCIIRune-8                           1.0950n ± 1%   0.2140n ± 0%  -80.46% (p=0.000 n=20)
EncodeSpanishRune-8                          1.499n ± 0%    1.414n ± 2%   -5.64% (p=0.000 n=20)
EncodeJapaneseRune-8                         1.960n ± 2%    1.716n ± 4%  -12.43% (p=0.000 n=20)
EncodeMaxRune-8                              2.145n ± 2%    2.227n ± 1%   +3.78% (p=0.000 n=20)
EncodeInvalidRuneMaxPlusOne-8                1.955n ± 2%    1.802n ± 2%   -7.80% (p=0.000 n=20)
EncodeInvalidRuneSurrogate-8                 1.946n ± 3%    1.777n ± 2%   -8.68% (p=0.000 n=20)
EncodeInvalidRuneNegative-8                  1.968n ± 2%    1.766n ± 2%  -10.29% (p=0.000 n=20)
geomean                                      1.757n         1.308n       -25.57%

Fixes #68131

Change-Id: Ibcafa75d63cca07a2e78cd06f6f1e382cb8c716e
Reviewed-on: https://go-review.googlesource.com/c/go/+/594115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>