For backend concurrency safety. Follow-up to CL 38721.

This does introduce a Nodes where there wasn't one before,
but these are so rare that the performance impact is negligible.

Does not pass toolstash-check, but the only change is line numbers,
and the new line numbers appear preferable.

Updates #15756

name       old alloc/op    new alloc/op    delta
Template      39.9MB ± 0%     39.9MB ± 0%    ~     (p=0.841 n=5+5)
Unicode       29.8MB ± 0%     29.8MB ± 0%    ~     (p=0.690 n=5+5)
GoTypes        113MB ± 0%      113MB ± 0%  +0.09%  (p=0.008 n=5+5)
SSA            854MB ± 0%      855MB ± 0%    ~     (p=0.222 n=5+5)
Flate         25.3MB ± 0%     25.3MB ± 0%    ~     (p=0.690 n=5+5)
GoParser      31.8MB ± 0%     31.9MB ± 0%    ~     (p=0.421 n=5+5)
Reflect       78.2MB ± 0%     78.3MB ± 0%    ~     (p=0.548 n=5+5)
Tar           26.7MB ± 0%     26.7MB ± 0%    ~     (p=0.690 n=5+5)
XML           42.3MB ± 0%     42.3MB ± 0%    ~     (p=0.222 n=5+5)

name       old allocs/op   new allocs/op   delta
Template        391k ± 1%       391k ± 0%    ~     (p=0.841 n=5+5)
Unicode         320k ± 0%       320k ± 0%    ~     (p=0.841 n=5+5)
GoTypes        1.14M ± 0%      1.14M ± 0%  +0.26%  (p=0.008 n=5+5)
SSA            7.60M ± 0%      7.60M ± 0%    ~     (p=0.548 n=5+5)
Flate           234k ± 0%       234k ± 1%    ~     (p=1.000 n=5+5)
GoParser        316k ± 1%       317k ± 0%    ~     (p=0.841 n=5+5)
Reflect         979k ± 0%       980k ± 0%    ~     (p=0.690 n=5+5)
Tar             251k ± 1%       251k ± 0%    ~     (p=0.595 n=5+5)
XML             394k ± 0%       393k ± 0%    ~     (p=0.222 n=5+5)


Change-Id: I237ae5502db4560f78ce021dc62f6d289797afd6
Reviewed-on: https://go-review.googlesource.com/39197
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>