encoding/{base32,base64}: speed up Encode

This CL clarifies (*Encoding).Encode and speeds it up by reducing the
number of bounds checks in its loop.

Here are some benchmark results (no change to allocations):

goos: darwin
goarch: arm64
pkg: encoding/base32
cpu: Apple M4
                  │     old     │             new              │
                  │   sec/op    │   sec/op     vs base         │
EncodeToString-10   7.310µ ± 0%   5.308µ ± 0%  -27.39% (n=180)
Encode-10           5.651µ ± 0%   3.603µ ± 0%  -36.25% (n=180)
geomean             6.427µ        4.373µ       -31.96%

                  │     old      │                  new                  │
                  │     B/s      │     B/s       vs base                 │
EncodeToString-10   1.044Gi ± 0%   1.437Gi ± 0%  +37.71% (p=0.000 n=180)
Encode-10           1.350Gi ± 0%   2.118Gi ± 0%  +56.88% (p=0.000 n=180)
geomean             1.187Gi        1.745Gi       +46.98%

pkg: encoding/base64
                  │     old     │             new              │
                  │   sec/op    │   sec/op     vs base         │
EncodeToString-10   7.058µ ± 0%   6.034µ ± 0%  -14.51% (n=180)

                  │     old      │                  new                  │
                  │     B/s      │     B/s       vs base                 │
EncodeToString-10   1.081Gi ± 0%   1.264Gi ± 0%  +16.97% (p=0.000 n=180)

Updates #20206

Change-Id: I7d46891ddb4371df004bfd612a8efc6638715b94
GitHub-Last-Rev: 1caac3d65532fefacbbed57f11a4a49273f173e2
GitHub-Pull-Request: golang/go#78344
Reviewed-on: https://go-review.googlesource.com/c/go/+/759100
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions