Don't bother with BenchmarkDecoderStream — it's doing something subtle
with the input buffer that isn't easy to replicate in a parallel test.

Results remain comparable with the non-parallel version with -cpu=1:

benchmark                          old ns/op     new ns/op     delta
BenchmarkCodeEncoder               22815832      21058729      -7.70%
BenchmarkCodeEncoder-6             22190561      3579757       -83.87%
BenchmarkCodeMarshal               25356621      25396429      +0.16%
BenchmarkCodeMarshal-6             25359813      4944908       -80.50%
BenchmarkCodeDecoder               94794556      88016360      -7.15%
BenchmarkCodeDecoder-6             93795028      16726283      -82.17%
BenchmarkDecoderStream             532           583           +9.59%
BenchmarkDecoderStream-6           598           550           -8.03%
BenchmarkCodeUnmarshal             97644168      89162504      -8.69%
BenchmarkCodeUnmarshal-6           96615302      17036419      -82.37%
BenchmarkCodeUnmarshalReuse        91747073      90298479      -1.58%
BenchmarkCodeUnmarshalReuse-6      89397165      15518005      -82.64%
BenchmarkUnmarshalString           808           843           +4.33%
BenchmarkUnmarshalString-6         912           220           -75.88%
BenchmarkUnmarshalFloat64          695           732           +5.32%
BenchmarkUnmarshalFloat64-6        710           191           -73.10%
BenchmarkUnmarshalInt64            635           640           +0.79%
BenchmarkUnmarshalInt64-6          618           185           -70.06%
BenchmarkIssue10335                916           947           +3.38%
BenchmarkIssue10335-6              879           216           -75.43%
BenchmarkNumberIsValid             34.7          34.3          -1.15%
BenchmarkNumberIsValid-6           34.9          36.7          +5.16%
BenchmarkNumberIsValidRegexp       1174          1121          -4.51%
BenchmarkNumberIsValidRegexp-6     1134          1119          -1.32%
BenchmarkSkipValue                 20506938      20708060      +0.98%
BenchmarkSkipValue-6               21627665      22375630      +3.46%
BenchmarkEncoderEncode             690           726           +5.22%
BenchmarkEncoderEncode-6           649           157           -75.81%

benchmark                    old MB/s     new MB/s     speedup
BenchmarkCodeEncoder         85.05        92.15        1.08x
BenchmarkCodeEncoder-6       87.45        542.07       6.20x
BenchmarkCodeMarshal         76.53        76.41        1.00x
BenchmarkCodeMarshal-6       76.52        392.42       5.13x
BenchmarkCodeDecoder         20.47        22.05        1.08x
BenchmarkCodeDecoder-6       20.69        116.01       5.61x
BenchmarkCodeUnmarshal       19.87        21.76        1.10x
BenchmarkCodeUnmarshal-6     20.08        113.90       5.67x
BenchmarkSkipValue           90.55        89.67        0.99x
BenchmarkSkipValue-6         90.83        87.80        0.97x

benchmark                    old allocs     new allocs     delta
BenchmarkIssue10335          4              4              +0.00%
BenchmarkIssue10335-6        4              4              +0.00%
BenchmarkEncoderEncode       1              1              +0.00%
BenchmarkEncoderEncode-6     1              1              +0.00%

benchmark                    old bytes     new bytes     delta
BenchmarkIssue10335          320           320           +0.00%
BenchmarkIssue10335-6        320           320           +0.00%
BenchmarkEncoderEncode       8             8             +0.00%
BenchmarkEncoderEncode-6     8             8             +0.00%

updates #18177

Change-Id: Ia4f5bf5ac0afbadb1705ed9f9e1b39dabba67b40
Reviewed-on: https://go-review.googlesource.com/36724Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>