compress/lzw: clarify code invariants

compress/lzw: clarify code invariants

This follows on from https://go-review.googlesource.com/c/go/+/191358
which was submitted as a comment-only change.

Benchmarks don't show any significant change:

compress/lzw
name            old speed      new speed      delta
Decoder/1e4-56  92.8MB/s ± 1%  92.7MB/s ± 1%   ~     (p=1.000 n=5+5)
Decoder/1e5-56   100MB/s ± 1%   100MB/s ± 1%   ~     (p=0.746 n=5+5)
Decoder/1e6-56   101MB/s ± 1%   101MB/s ± 1%   ~     (p=0.381 n=5+5)

image/gif
name                old speed      new speed      delta
Decode-56           63.2MB/s ± 1%  63.2MB/s ± 1%   ~     (p=0.690 n=5+5)

Change-Id: Ic36b5410cb06ca258da32e40da1f1ff6c44cff86
Reviewed-on: https://go-review.googlesource.com/c/go/+/194938
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>