Emitting it here instead of rewriting the tree earlier sets us up
to generate an inline check, like we do for single pointers.
But even without the inline check, generating at this level lets
us generate significantly more efficient code, probably due to
having fewer temporaries and less complex high-level code
for the compiler to churn through.

Revcomp is worse, almost certainly due to register pressure.

name                                       old                     new          delta
BenchmarkBinaryTree17              18.0s × (0.99,1.01)     18.0s × (0.99,1.01)  ~
BenchmarkFannkuch11                4.43s × (1.00,1.00)     4.36s × (1.00,1.00)  -1.44%
BenchmarkFmtFprintfEmpty           114ns × (0.95,1.05)      86ns × (0.97,1.06)  -24.12%
BenchmarkFmtFprintfString          468ns × (0.99,1.01)     420ns × (0.99,1.02)  -10.16%
BenchmarkFmtFprintfInt             433ns × (1.00,1.01)     386ns × (0.99,1.02)  -10.74%
BenchmarkFmtFprintfIntInt          748ns × (0.99,1.01)     647ns × (0.99,1.01)  -13.56%
BenchmarkFmtFprintfPrefixedInt     547ns × (0.99,1.01)     499ns × (0.99,1.02)  -8.78%
BenchmarkFmtFprintfFloat           756ns × (1.00,1.01)     689ns × (1.00,1.00)  -8.86%
BenchmarkFmtManyArgs              2.79µs × (1.00,1.01)    2.53µs × (1.00,1.00)  -9.30%
BenchmarkGobDecode                39.6ms × (0.99,1.00)    39.2ms × (0.98,1.01)  -1.07%
BenchmarkGobEncode                37.6ms × (1.00,1.01)    37.5ms × (0.99,1.01)  ~
BenchmarkGzip                      663ms × (0.99,1.02)     660ms × (0.98,1.01)  ~
BenchmarkGunzip                    142ms × (1.00,1.00)     143ms × (1.00,1.00)  ~
BenchmarkHTTPClientServer          132µs × (0.99,1.01)     133µs × (0.99,1.02)  ~
BenchmarkJSONEncode               56.2ms × (0.99,1.01)    54.0ms × (0.98,1.01)  -3.97%
BenchmarkJSONDecode                138ms × (1.00,1.00)     134ms × (0.99,1.02)  -2.70%
BenchmarkMandelbrot200            6.03ms × (1.00,1.01)    6.00ms × (1.00,1.01)  ~
BenchmarkGoParse                  9.82ms × (0.93,1.10)   10.35ms × (0.88,1.11)  ~
BenchmarkRegexpMatchEasy0_32       207ns × (1.00,1.00)     163ns × (0.99,1.01)  -21.26%
BenchmarkRegexpMatchEasy0_1K       581ns × (1.00,1.01)     566ns × (0.99,1.00)  -2.50%
BenchmarkRegexpMatchEasy1_32       185ns × (0.99,1.01)     138ns × (1.00,1.01)  -25.41%
BenchmarkRegexpMatchEasy1_1K       975ns × (1.00,1.01)     892ns × (1.00,1.00)  -8.51%
BenchmarkRegexpMatchMedium_32      328ns × (0.99,1.00)     252ns × (1.00,1.00)  -23.17%
BenchmarkRegexpMatchMedium_1K     88.6µs × (1.00,1.01)    73.0µs × (1.00,1.01)  -17.66%
BenchmarkRegexpMatchHard_32       4.69µs × (0.95,1.03)    3.85µs × (1.00,1.01)  -17.91%
BenchmarkRegexpMatchHard_1K        133µs × (1.00,1.01)     117µs × (1.00,1.00)  -12.34%
BenchmarkRevcomp                   902ms × (0.99,1.05)    1001ms × (0.94,1.01)  +11.04%
BenchmarkTemplate                  174ms × (0.99,1.01)     160ms × (0.99,1.01)  -7.70%
BenchmarkTimeParse                 639ns × (1.00,1.00)     622ns × (1.00,1.00)  -2.66%
BenchmarkTimeFormat                736ns × (1.00,1.01)     736ns × (1.00,1.02)  ~

Change-Id: Ib3bbeb379f5f4819e6f5dcf69bc88a2b7ed41460
Reviewed-on: https://go-review.googlesource.com/9225Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>