Reintroduce an optimization discarded during the initial conversion
from 4-bit heap bitmaps to 2-bit heap bitmaps: when we reach the
place in the bitmap where there are no more pointers, mark that position
for the GC so that it can avoid scanning past that place.

During heapBitsSetType we can also avoid initializing heap bitmap
beyond that location, which gives a bit of a win compared to Go 1.4.
This particular optimization (not initializing the heap bitmap) may not last:
we might change typedmemmove to use the heap bitmap, in which
case it would all need to be initialized. The early stop in the GC scan
will stay no matter what.

Compared to Go 1.4 (github.com/rsc/go, branch go14bench):
name                    old mean              new mean              delta
SetTypeNode64           80.7ns × (1.00,1.01)  57.4ns × (1.00,1.01)  -28.83% (p=0.000)
SetTypeNode64Dead       80.5ns × (1.00,1.01)  13.1ns × (0.99,1.02)  -83.77% (p=0.000)
SetTypeNode64Slice      2.16µs × (1.00,1.01)  1.54µs × (1.00,1.01)  -28.75% (p=0.000)
SetTypeNode64DeadSlice  2.16µs × (1.00,1.01)  1.52µs × (1.00,1.00)  -29.74% (p=0.000)

Compared to previous CL:
name                    old mean              new mean              delta
SetTypeNode64           56.7ns × (1.00,1.00)  57.4ns × (1.00,1.01)   +1.19% (p=0.000)
SetTypeNode64Dead       57.2ns × (1.00,1.00)  13.1ns × (0.99,1.02)  -77.15% (p=0.000)
SetTypeNode64Slice      1.56µs × (1.00,1.01)  1.54µs × (1.00,1.01)   -0.89% (p=0.000)
SetTypeNode64DeadSlice  1.55µs × (1.00,1.01)  1.52µs × (1.00,1.00)   -2.23% (p=0.000)

This is the last CL in the sequence converting from the 4-bit heap
to the 2-bit heap, with all the same optimizations reenabled.
Compared to before that process began (compared to CL 9701 patch set 1):

name                    old mean              new mean              delta
BinaryTree17             5.87s × (0.94,1.09)   5.91s × (0.96,1.06)    ~    (p=0.578)
Fannkuch11               4.32s × (1.00,1.00)   4.32s × (1.00,1.00)    ~    (p=0.474)
FmtFprintfEmpty         89.1ns × (0.95,1.16)  89.0ns × (0.93,1.10)    ~    (p=0.942)
FmtFprintfString         283ns × (0.98,1.02)   298ns × (0.98,1.06)  +5.33% (p=0.000)
FmtFprintfInt            284ns × (0.98,1.04)   286ns × (0.98,1.03)    ~    (p=0.208)
FmtFprintfIntInt         486ns × (0.98,1.03)   498ns × (0.97,1.06)  +2.48% (p=0.000)
FmtFprintfPrefixedInt    400ns × (0.99,1.02)   408ns × (0.98,1.02)  +2.23% (p=0.000)
FmtFprintfFloat          566ns × (0.99,1.01)   587ns × (0.98,1.01)  +3.69% (p=0.000)
FmtManyArgs             1.91µs × (0.99,1.02)  1.94µs × (0.99,1.02)  +1.81% (p=0.000)
GobDecode               15.5ms × (0.98,1.05)  15.8ms × (0.98,1.03)  +1.94% (p=0.002)
GobEncode               11.9ms × (0.97,1.03)  12.0ms × (0.96,1.09)    ~    (p=0.263)
Gzip                     648ms × (0.99,1.01)   648ms × (0.99,1.01)    ~    (p=0.992)
Gunzip                   143ms × (1.00,1.00)   143ms × (1.00,1.01)    ~    (p=0.585)
HTTPClientServer        89.2µs × (0.99,1.02)  90.3µs × (0.98,1.01)  +1.24% (p=0.000)
JSONEncode              32.3ms × (0.97,1.06)  31.6ms × (0.99,1.01)  -2.29% (p=0.000)
JSONDecode               106ms × (0.99,1.01)   107ms × (1.00,1.01)  +0.62% (p=0.000)
Mandelbrot200           6.02ms × (1.00,1.00)  6.03ms × (1.00,1.01)    ~    (p=0.250)
GoParse                 6.57ms × (0.97,1.06)  6.53ms × (0.99,1.03)    ~    (p=0.243)
RegexpMatchEasy0_32      162ns × (1.00,1.00)   161ns × (1.00,1.01)  -0.80% (p=0.000)
RegexpMatchEasy0_1K      561ns × (0.99,1.02)   541ns × (0.99,1.01)  -3.67% (p=0.000)
RegexpMatchEasy1_32      145ns × (0.95,1.04)   138ns × (1.00,1.00)  -5.04% (p=0.000)
RegexpMatchEasy1_1K      864ns × (0.99,1.04)   887ns × (0.99,1.01)  +2.57% (p=0.000)
RegexpMatchMedium_32     255ns × (0.99,1.04)   253ns × (0.99,1.01)  -1.05% (p=0.012)
RegexpMatchMedium_1K    73.9µs × (0.98,1.04)  72.8µs × (1.00,1.00)  -1.51% (p=0.005)
RegexpMatchHard_32      3.92µs × (0.98,1.04)  3.85µs × (1.00,1.01)  -1.88% (p=0.002)
RegexpMatchHard_1K       120µs × (0.98,1.04)   117µs × (1.00,1.01)  -2.02% (p=0.001)
Revcomp                  936ms × (0.95,1.08)   922ms × (0.97,1.08)    ~    (p=0.234)
Template                 130ms × (0.98,1.04)   126ms × (0.99,1.01)  -2.99% (p=0.000)
TimeParse                638ns × (0.98,1.05)   628ns × (0.99,1.01)  -1.54% (p=0.004)
TimeFormat               674ns × (0.99,1.01)   668ns × (0.99,1.01)  -0.80% (p=0.001)

The slowdown of the first few benchmarks seems to be due to the new
atomic operations for certain small size allocations. But the larger
benchmarks mostly improve, probably due to the decreased memory
pressure from having half as much heap bitmap.

CL 9706, which removes the (never used anymore) wbshadow mode,
gets back what is lost in the early microbenchmarks.

Change-Id: I37423a209e8ec2a2e92538b45cac5422a6acd32d
Reviewed-on: https://go-review.googlesource.com/9705Reviewed-by: Rick Hudson <rlh@golang.org>