I believe the benchmarks that get slower are under register pressure,
and not making the call unconditionally makes the pressure worse,
and the register allocator doesn't do a great job. But part of the point
of this sequence is to get the write barriers out of the way so I can work
on the register allocator, so that's okay.

name                                       old                     new          delta
BenchmarkBinaryTree17              17.9s × (1.00,1.01)     18.0s × (0.99,1.01)  ~
BenchmarkFannkuch11                4.43s × (1.00,1.00)     4.43s × (1.00,1.00)  ~
BenchmarkFmtFprintfEmpty           110ns × (1.00,1.06)     114ns × (0.95,1.05)  ~
BenchmarkFmtFprintfString          487ns × (0.99,1.00)     468ns × (0.99,1.01)  -4.00%
BenchmarkFmtFprintfInt             450ns × (0.99,1.00)     433ns × (1.00,1.01)  -3.88%
BenchmarkFmtFprintfIntInt          762ns × (1.00,1.00)     748ns × (0.99,1.01)  -1.84%
BenchmarkFmtFprintfPrefixedInt     584ns × (0.99,1.01)     547ns × (0.99,1.01)  -6.26%
BenchmarkFmtFprintfFloat           738ns × (1.00,1.00)     756ns × (1.00,1.01)  +2.37%
BenchmarkFmtManyArgs              2.80µs × (1.00,1.01)    2.79µs × (1.00,1.01)  ~
BenchmarkGobDecode                39.0ms × (0.99,1.00)    39.6ms × (0.99,1.00)  +1.54%
BenchmarkGobEncode                37.8ms × (0.98,1.01)    37.6ms × (1.00,1.01)  ~
BenchmarkGzip                      661ms × (0.99,1.01)     663ms × (0.99,1.02)  ~
BenchmarkGunzip                    142ms × (1.00,1.00)     142ms × (1.00,1.00)  ~
BenchmarkHTTPClientServer          132µs × (0.99,1.01)     132µs × (0.99,1.01)  ~
BenchmarkJSONEncode               56.3ms × (0.99,1.01)    56.2ms × (0.99,1.01)  ~
BenchmarkJSONDecode                138ms × (0.99,1.01)     138ms × (1.00,1.00)  ~
BenchmarkMandelbrot200            6.01ms × (1.00,1.00)    6.03ms × (1.00,1.01)  +0.23%
BenchmarkGoParse                  10.2ms × (0.87,1.05)     9.8ms × (0.93,1.10)  ~
BenchmarkRegexpMatchEasy0_32       208ns × (1.00,1.00)     207ns × (1.00,1.00)  ~
BenchmarkRegexpMatchEasy0_1K       588ns × (1.00,1.00)     581ns × (1.00,1.01)  -1.27%
BenchmarkRegexpMatchEasy1_32       182ns × (0.99,1.01)     185ns × (0.99,1.01)  +1.65%
BenchmarkRegexpMatchEasy1_1K       986ns × (1.00,1.01)     975ns × (1.00,1.01)  -1.17%
BenchmarkRegexpMatchMedium_32      323ns × (1.00,1.01)     328ns × (0.99,1.00)  +1.55%
BenchmarkRegexpMatchMedium_1K     89.9µs × (1.00,1.00)    88.6µs × (1.00,1.01)  -1.38%
BenchmarkRegexpMatchHard_32       4.72µs × (0.95,1.01)    4.69µs × (0.95,1.03)  ~
BenchmarkRegexpMatchHard_1K        133µs × (1.00,1.01)     133µs × (1.00,1.01)  ~
BenchmarkRevcomp                   900ms × (1.00,1.05)     902ms × (0.99,1.05)  ~
BenchmarkTemplate                  168ms × (0.99,1.01)     174ms × (0.99,1.01)  +3.30%
BenchmarkTimeParse                 637ns × (1.00,1.00)     639ns × (1.00,1.00)  +0.31%
BenchmarkTimeFormat                738ns × (1.00,1.00)     736ns × (1.00,1.01)  ~

Change-Id: I03ce152852edec404538f6c20eb650fac82e2aa2
Reviewed-on: https://go-review.googlesource.com/9224Reviewed-by: Austin Clements <austin@google.com>