Most of the encoding time is spent in the first Encode loop, since the
rest of the function only deals with the few remaining bytes. Any
unnecessary work done in that loop body matters tremendously.

One such unnecessary bottleneck was the use of the enc.encode table.
Since enc is a pointer receiver, and the field is first used within the
loop, the encoder must perform a nil check at every iteration.

Add a dummy use of the field before the start of the loop, to move the
nil check there. After that line, the compiler now knows that enc can't
be nil, and thus the hot loop is free of nil checks.

name              old time/op    new time/op    delta
EncodeToString-4    14.7µs ± 0%    13.7µs ± 1%  -6.53%  (p=0.000 n=10+10)

name              old speed      new speed      delta
EncodeToString-4   559MB/s ± 0%   598MB/s ± 1%  +6.99%  (p=0.000 n=10+10)

Updates #20206.

Change-Id: Icbb523a7bd9e470a8be0a448d1d78ade97ed4ff6
Reviewed-on: https://go-review.googlesource.com/c/151158
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>