It's no longer necessary to maintain the bitmap of noscan objects
since we now use the span metadata to determine that they're noscan
instead of the bitmap.

The combined effect of segregating noscan spans and the follow-on
optimizations is roughly a 1% improvement in performance across the
go1 benchmarks and the x/benchmarks, with no increase in heap size.

Benchmark details: https://perf.golang.org/search?q=upload:20170420.1

name                       old time/op    new time/op    delta
Garbage/benchmem-MB=64-12    2.27ms ± 0%    2.25ms ± 1%  -0.96% (p=0.000 n=15+18)

name                      old time/op    new time/op    delta
BinaryTree17-12              2.53s ± 2%     2.55s ± 1%  +0.68%        (p=0.001 n=17+16)
Fannkuch11-12                3.02s ± 0%     3.01s ± 0%  -0.15%        (p=0.000 n=16+16)
FmtFprintfEmpty-12          47.1ns ± 7%    47.0ns ± 5%    ~           (p=0.886 n=20+17)
FmtFprintfString-12         73.6ns ± 3%    73.8ns ± 1%  +0.30%        (p=0.026 n=19+17)
FmtFprintfInt-12            80.3ns ± 2%    80.2ns ± 1%    ~           (p=0.994 n=20+18)
FmtFprintfIntInt-12          124ns ± 0%     124ns ± 0%    ~     (all samples are equal)
FmtFprintfPrefixedInt-12     172ns ± 1%     171ns ± 1%  -0.72%        (p=0.003 n=20+18)
FmtFprintfFloat-12           217ns ± 1%     216ns ± 1%  -0.27%        (p=0.019 n=18+19)
FmtManyArgs-12               490ns ± 1%     488ns ± 0%  -0.36%        (p=0.014 n=18+18)
GobDecode-12                6.71ms ± 1%    6.73ms ± 1%  +0.42%        (p=0.000 n=20+20)
GobEncode-12                5.25ms ± 0%    5.24ms ± 0%  -0.20%        (p=0.001 n=18+20)
Gzip-12                      227ms ± 0%     226ms ± 1%    ~           (p=0.107 n=20+19)
Gunzip-12                   38.8ms ± 0%    38.8ms ± 0%    ~           (p=0.221 n=19+18)
HTTPClientServer-12         75.4µs ± 1%    76.3µs ± 1%  +1.26%        (p=0.000 n=20+19)
JSONEncode-12               14.7ms ± 0%    14.7ms ± 1%  -0.14%        (p=0.002 n=18+17)
JSONDecode-12               57.6ms ± 0%    55.2ms ± 0%  -4.13%        (p=0.000 n=19+19)
Mandelbrot200-12            3.73ms ± 0%    3.73ms ± 0%  -0.09%        (p=0.000 n=19+17)
GoParse-12                  3.18ms ± 1%    3.15ms ± 1%  -0.90%        (p=0.000 n=18+20)
RegexpMatchEasy0_32-12      73.3ns ± 2%    73.2ns ± 1%    ~           (p=0.994 n=20+18)
RegexpMatchEasy0_1K-12       236ns ± 2%     234ns ± 1%  -0.70%        (p=0.002 n=19+17)
RegexpMatchEasy1_32-12      69.7ns ± 2%    69.9ns ± 2%    ~           (p=0.416 n=20+20)
RegexpMatchEasy1_1K-12       366ns ± 1%     365ns ± 1%    ~           (p=0.376 n=19+17)
RegexpMatchMedium_32-12      109ns ± 1%     108ns ± 1%    ~           (p=0.461 n=17+18)
RegexpMatchMedium_1K-12     35.2µs ± 1%    35.2µs ± 3%    ~           (p=0.238 n=19+20)
RegexpMatchHard_32-12       1.77µs ± 1%    1.77µs ± 1%  +0.33%        (p=0.007 n=17+16)
RegexpMatchHard_1K-12       53.2µs ± 0%    53.3µs ± 0%  +0.26%        (p=0.001 n=17+17)
Revcomp-12                  1.13s ±117%    0.87s ±184%    ~           (p=0.813 n=20+19)
Template-12                 63.9ms ± 1%    64.6ms ± 1%  +1.18%        (p=0.000 n=19+20)
TimeParse-12                 313ns ± 5%     312ns ± 0%    ~           (p=0.114 n=20+19)
TimeFormat-12                336ns ± 0%     333ns ± 0%  -0.97%        (p=0.000 n=18+16)
[Geo mean]                  50.6µs         50.1µs       -1.04%

This is a cherry-pick of dev.garbage commit edb54c300f, with updated
benchmark results.

Change-Id: Ic77faaa15cdac3bfbbb0032dde5c204e05a0fd8e
Reviewed-on: https://go-review.googlesource.com/41253
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>