encoding/base64: improve performance up to 20% total

Improve base64 encoding/decoding performance by
suppressing compiler boundary checks on decode.

name                 old speed      new speed      delta
EncodeToString-8      570MB/s ± 1%   573MB/s ± 1%     ~     (p=0.421 n=5+5)
DecodeString/2-8     88.6MB/s ± 3%  91.6MB/s ± 2%   +3.37%  (p=0.016 n=5+5)
DecodeString/4-8      162MB/s ± 1%   168MB/s ± 0%   +4.12%  (p=0.008 n=5+5)
DecodeString/8-8      203MB/s ± 0%   214MB/s ± 0%   +5.18%  (p=0.008 n=5+5)
DecodeString/64-8     471MB/s ± 1%   520MB/s ± 1%  +10.50%  (p=0.008 n=5+5)
DecodeString/8192-8   757MB/s ± 0%   895MB/s ± 1%  +18.29%  (p=0.008 n=5+5)

Change-Id: I135243c11aa4c974a4a4e95c5c2abb0635d52c8c
GitHub-Last-Rev: 2c87abcb287f4459888941ae2633edcd8eb5b79d
GitHub-Pull-Request: golang/go#36910
Reviewed-on: https://go-review.googlesource.com/c/go/+/217117


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>