scanobject with ptrmask!=nil is only ever called with the base
pointer of a heap object. Currently, scanobject calls
heapBitsForObject, which goes to a great deal of trouble to check
that the pointer points into the heap and to find the base of the
object it points to, both of which are completely unnecessary in
this case.

Replace this call to heapBitsForObject with much simpler logic to
fetch the span and compute the heap bits.

Benchmark results with five runs:

name                                    old mean                new mean        delta
BenchmarkBinaryTree17              9.21s × (0.95,1.02)     8.55s × (0.91,1.03)  -7.16% (p=0.022)
BenchmarkFannkuch11                2.65s × (1.00,1.00)     2.62s × (1.00,1.00)  -1.10% (p=0.000)
BenchmarkFmtFprintfEmpty          73.2ns × (0.99,1.01)    71.7ns × (1.00,1.01)  -1.99% (p=0.004)
BenchmarkFmtFprintfString          302ns × (0.99,1.00)     292ns × (0.98,1.02)  -3.31% (p=0.020)
BenchmarkFmtFprintfInt             281ns × (0.98,1.01)     279ns × (0.96,1.02)  ~ (p=0.596)
BenchmarkFmtFprintfIntInt          482ns × (0.98,1.01)     488ns × (0.95,1.02)  ~ (p=0.419)
BenchmarkFmtFprintfPrefixedInt     382ns × (0.99,1.01)     365ns × (0.96,1.02)  -4.35% (p=0.015)
BenchmarkFmtFprintfFloat           475ns × (0.99,1.01)     472ns × (1.00,1.00)  ~ (p=0.108)
BenchmarkFmtManyArgs              1.89µs × (1.00,1.01)    1.90µs × (0.94,1.02)  ~ (p=0.883)
BenchmarkGobDecode                22.4ms × (0.99,1.01)    21.9ms × (0.92,1.04)  ~ (p=0.332)
BenchmarkGobEncode                24.7ms × (0.98,1.02)    23.9ms × (0.87,1.07)  ~ (p=0.407)
BenchmarkGzip                      397ms × (0.99,1.01)     398ms × (0.99,1.01)  ~ (p=0.718)
BenchmarkGunzip                   96.7ms × (1.00,1.00)    96.9ms × (1.00,1.00)  ~ (p=0.230)
BenchmarkHTTPClientServer         71.5µs × (0.98,1.01)    68.5µs × (0.92,1.06)  ~ (p=0.243)
BenchmarkJSONEncode               46.1ms × (0.98,1.01)    44.9ms × (0.98,1.03)  -2.51% (p=0.040)
BenchmarkJSONDecode               86.1ms × (0.99,1.01)    86.5ms × (0.99,1.01)  ~ (p=0.343)
BenchmarkMandelbrot200            4.12ms × (1.00,1.00)    4.13ms × (1.00,1.00)  +0.23% (p=0.000)
BenchmarkGoParse                  5.89ms × (0.96,1.03)    5.82ms × (0.96,1.04)  ~ (p=0.522)
BenchmarkRegexpMatchEasy0_32       141ns × (0.99,1.01)     142ns × (1.00,1.00)  ~ (p=0.178)
BenchmarkRegexpMatchEasy0_1K       408ns × (1.00,1.00)     392ns × (0.99,1.00)  -3.83% (p=0.000)
BenchmarkRegexpMatchEasy1_32       122ns × (1.00,1.00)     122ns × (1.00,1.00)  ~ (p=0.178)
BenchmarkRegexpMatchEasy1_1K       626ns × (1.00,1.01)     624ns × (0.99,1.00)  ~ (p=0.122)
BenchmarkRegexpMatchMedium_32      202ns × (0.99,1.00)     205ns × (0.99,1.01)  +1.58% (p=0.001)
BenchmarkRegexpMatchMedium_1K     54.4µs × (1.00,1.00)    55.5µs × (1.00,1.00)  +1.86% (p=0.000)
BenchmarkRegexpMatchHard_32       2.68µs × (1.00,1.00)    2.71µs × (1.00,1.00)  +0.97% (p=0.002)
BenchmarkRegexpMatchHard_1K       79.8µs × (1.00,1.01)    80.5µs × (1.00,1.01)  +0.94% (p=0.003)
BenchmarkRevcomp                   590ms × (0.99,1.01)     585ms × (1.00,1.00)  ~ (p=0.066)
BenchmarkTemplate                  111ms × (0.97,1.02)     112ms × (0.99,1.01)  ~ (p=0.201)
BenchmarkTimeParse                 392ns × (1.00,1.00)     385ns × (1.00,1.00)  -1.69% (p=0.000)
BenchmarkTimeFormat                449ns × (0.98,1.01)     448ns × (0.99,1.01)  ~ (p=0.550)

Change-Id: Ie7c3830c481d96c9043e7bf26853c6c1d05dc9f4
Reviewed-on: https://go-review.googlesource.com/9364Reviewed-by: Rick Hudson <rlh@golang.org>