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 almost no effect on the go1 benchmarks and a 1.19%
improvement in the garbage benchmark:

name              old time/op  new time/op  delta
XBenchGarbage-12  2.13ms ± 2%  2.11ms ± 1%  -1.19%  (p=0.000 n=19+20)

name                      old time/op    new time/op    delta
BinaryTree17-12              2.42s ± 1%     2.41s ± 1%  -0.50%  (p=0.001 n=20+17)
Fannkuch11-12                2.14s ± 0%     2.12s ± 0%  -0.91%  (p=0.000 n=20+17)
FmtFprintfEmpty-12          45.2ns ± 0%    45.2ns ± 1%    ~     (p=0.677 n=16+19)
FmtFprintfString-12          131ns ± 0%     132ns ± 1%  +0.57%  (p=0.000 n=16+20)
FmtFprintfInt-12             126ns ± 1%     126ns ± 0%    ~     (p=0.078 n=18+16)
FmtFprintfIntInt-12          199ns ± 0%     195ns ± 0%  -2.19%  (p=0.000 n=14+20)
FmtFprintfPrefixedInt-12     196ns ± 1%     196ns ± 0%    ~     (p=0.155 n=19+16)
FmtFprintfFloat-12           254ns ± 0%     253ns ± 0%  -0.50%  (p=0.000 n=14+19)
FmtManyArgs-12               803ns ± 1%     798ns ± 0%  -0.71%  (p=0.000 n=18+19)
GobDecode-12                7.11ms ± 1%    7.07ms ± 1%  -0.50%  (p=0.024 n=18+19)
GobEncode-12                5.87ms ± 0%    5.86ms ± 1%    ~     (p=0.113 n=19+20)
Gzip-12                      218ms ± 1%     218ms ± 1%    ~     (p=0.879 n=19+20)
Gunzip-12                   37.2ms ± 0%    37.3ms ± 0%  +0.14%  (p=0.047 n=19+20)
HTTPClientServer-12         80.5µs ± 6%    82.8µs ± 8%  +2.91%  (p=0.008 n=19+20)
JSONEncode-12               15.4ms ± 1%    15.4ms ± 1%  -0.32%  (p=0.003 n=18+19)
JSONDecode-12               55.1ms ± 1%    53.0ms ± 1%  -3.87%  (p=0.000 n=18+20)
Mandelbrot200-12            4.08ms ± 1%    4.10ms ± 1%  +0.34%  (p=0.001 n=19+17)
GoParse-12                  3.20ms ± 1%    3.21ms ± 1%    ~     (p=0.138 n=19+19)
RegexpMatchEasy0_32-12      70.6ns ± 2%    70.4ns ± 2%    ~     (p=0.343 n=20+20)
RegexpMatchEasy0_1K-12       240ns ± 0%     242ns ± 2%  +0.88%  (p=0.000 n=15+20)
RegexpMatchEasy1_32-12      70.5ns ± 1%    70.2ns ± 3%    ~     (p=0.053 n=18+20)
RegexpMatchEasy1_1K-12       374ns ± 1%     374ns ± 1%    ~     (p=0.705 n=20+19)
RegexpMatchMedium_32-12      108ns ± 1%     108ns ± 1%    ~     (p=0.854 n=20+19)
RegexpMatchMedium_1K-12     33.5µs ± 1%    33.6µs ± 2%    ~     (p=0.897 n=18+20)
RegexpMatchHard_32-12       1.76µs ± 1%    1.75µs ± 1%    ~     (p=0.771 n=18+18)
RegexpMatchHard_1K-12       52.8µs ± 1%    52.8µs ± 1%    ~     (p=0.678 n=17+19)
Revcomp-12                   381ms ± 1%     380ms ± 0%    ~     (p=0.320 n=20+16)
Template-12                 65.6ms ± 1%    65.1ms ± 2%  -0.75%  (p=0.003 n=20+20)
TimeParse-12                 324ns ± 1%     326ns ± 1%  +0.72%  (p=0.000 n=18+18)
TimeFormat-12                342ns ± 0%     343ns ± 1%  +0.22%  (p=0.004 n=15+18)
[Geo mean]                  52.4µs         52.3µs       -0.18%

Change-Id: Ic77faaa15cdac3bfbbb0032dde5c204e05a0fd8e
Reviewed-on: https://go-review.googlesource.com/23702Reviewed-by: Rick Hudson <rlh@golang.org>