It is not safe to modify Node.Opt in the backend.
Instead of using Node.Opt to store liveness variable indices, use a map.
This simplifies the code and makes it much more clearly race-free.
There are generally few such variables, so the maps are not a significant
source of allocations; this also remove some allocations from putting
int32s into interfaces.

Because map lookups are more expensive than interface value extraction,
reorder valueEffects to do the map lookup last.

The only remaining use of Node.Opt is now in esc.go.

Passes toolstash-check.

Fixes #20144

name        old alloc/op      new alloc/op      delta
Template         37.8MB ± 0%       37.9MB ± 0%    ~     (p=0.548 n=5+5)
Unicode          28.9MB ± 0%       28.9MB ± 0%    ~     (p=0.548 n=5+5)
GoTypes           110MB ± 0%        110MB ± 0%  +0.16%  (p=0.008 n=5+5)
Compiler          461MB ± 0%        462MB ± 0%  +0.08%  (p=0.008 n=5+5)
SSA              1.11GB ± 0%       1.11GB ± 0%  +0.11%  (p=0.008 n=5+5)
Flate            24.7MB ± 0%       24.7MB ± 0%    ~     (p=0.690 n=5+5)
GoParser         31.1MB ± 0%       31.1MB ± 0%    ~     (p=0.841 n=5+5)
Reflect          73.7MB ± 0%       73.8MB ± 0%  +0.23%  (p=0.008 n=5+5)
Tar              25.8MB ± 0%       25.7MB ± 0%    ~     (p=0.690 n=5+5)
XML              41.2MB ± 0%       41.2MB ± 0%    ~     (p=0.841 n=5+5)
[Geo mean]       71.9MB            71.9MB       +0.06%

name        old allocs/op     new allocs/op     delta
Template           385k ± 0%         384k ± 0%    ~     (p=0.548 n=5+5)
Unicode            344k ± 0%         343k ± 1%    ~     (p=0.421 n=5+5)
GoTypes           1.16M ± 0%        1.16M ± 0%    ~     (p=0.690 n=5+5)
Compiler          4.43M ± 0%        4.42M ± 0%    ~     (p=0.095 n=5+5)
SSA               9.86M ± 0%        9.84M ± 0%  -0.19%  (p=0.008 n=5+5)
Flate              238k ± 0%         238k ± 0%    ~     (p=1.000 n=5+5)
GoParser           321k ± 0%         320k ± 0%    ~     (p=0.310 n=5+5)
Reflect            956k ± 0%         956k ± 0%    ~     (p=1.000 n=5+5)
Tar                252k ± 0%         251k ± 0%    ~     (p=0.056 n=5+5)
XML                402k ± 1%         400k ± 1%  -0.57%  (p=0.032 n=5+5)
[Geo mean]         740k              739k       -0.19%

Change-Id: Id5916c9def76add272e89c59fe10968f0a6bb01d
Reviewed-on: https://go-review.googlesource.com/42135
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>