This simplifies the code a bit and provides a modest speedup for
Marshal with many CPUs.

updates #17973
updates #18177

name          old time/op    new time/op    delta
Marshal         15.8µs ± 1%    15.9µs ± 1%   +0.67%  (p=0.021 n=8+7)
Marshal-6       5.76µs ±11%    5.17µs ± 2%  -10.36%  (p=0.002 n=8+8)
Marshal-48      9.88µs ± 5%    7.31µs ± 6%  -26.04%  (p=0.000 n=8+8)
Unmarshal       44.7µs ± 3%    45.1µs ± 5%     ~     (p=0.645 n=8+8)
Unmarshal-6     12.1µs ± 7%    11.8µs ± 8%     ~     (p=0.442 n=8+8)
Unmarshal-48    18.7µs ± 3%    18.2µs ± 4%     ~     (p=0.054 n=7+8)

name          old alloc/op   new alloc/op   delta
Marshal         5.78kB ± 0%    5.78kB ± 0%     ~     (all equal)
Marshal-6       5.78kB ± 0%    5.78kB ± 0%     ~     (all equal)
Marshal-48      5.78kB ± 0%    5.78kB ± 0%     ~     (all equal)
Unmarshal       8.58kB ± 0%    8.58kB ± 0%     ~     (all equal)
Unmarshal-6     8.58kB ± 0%    8.58kB ± 0%     ~     (all equal)
Unmarshal-48    8.58kB ± 0%    8.58kB ± 0%     ~     (p=1.000 n=8+8)

name          old allocs/op  new allocs/op  delta
Marshal           23.0 ± 0%      23.0 ± 0%     ~     (all equal)
Marshal-6         23.0 ± 0%      23.0 ± 0%     ~     (all equal)
Marshal-48        23.0 ± 0%      23.0 ± 0%     ~     (all equal)
Unmarshal          189 ± 0%       189 ± 0%     ~     (all equal)
Unmarshal-6        189 ± 0%       189 ± 0%     ~     (all equal)
Unmarshal-48       189 ± 0%       189 ± 0%     ~     (all equal)

https://perf.golang.org/search?q=upload:20170427.5

Change-Id: I4ee95a99540d3e4e47e056fff18357efd2cd340a
Reviewed-on: https://go-review.googlesource.com/41991
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>