Though LSym.Version is an int, it can only have the value 0 or 1.
Using that, split Link.hash into two maps, one for version 0
(which is far more common) and one for version 1.
This lets use just the name for lookups,
which is both faster and more compact.
This matters because Link.hash map lookups are frequent,
and will be contended once the backend is concurrent.

name        old time/op       new time/op       delta
Template          194ms ± 3%        192ms ± 5%  -1.46%  (p=0.000 n=47+49)
Unicode          84.5ms ± 3%       83.8ms ± 3%  -0.81%  (p=0.011 n=50+49)
GoTypes           543ms ± 2%        545ms ± 4%    ~     (p=0.566 n=46+49)
Compiler          2.48s ± 2%        2.48s ± 3%    ~     (p=0.706 n=47+50)
SSA               5.94s ± 3%        5.98s ± 2%  +0.55%  (p=0.040 n=49+50)
Flate             119ms ± 6%        119ms ± 4%    ~     (p=0.681 n=48+47)
GoParser          145ms ± 4%        145ms ± 3%    ~     (p=0.662 n=47+49)
Reflect           348ms ± 3%        344ms ± 3%  -1.17%  (p=0.000 n=47+47)
Tar               105ms ± 4%        104ms ± 3%    ~     (p=0.155 n=50+47)
XML               197ms ± 2%        197ms ± 3%    ~     (p=0.666 n=49+49)
[Geo mean]        332ms             331ms       -0.37%

name        old user-time/op  new user-time/op  delta
Template          230ms ±10%        226ms ±10%  -1.85%  (p=0.041 n=50+50)
Unicode           104ms ± 6%        103ms ± 5%    ~     (p=0.076 n=49+49)
GoTypes           707ms ± 4%        705ms ± 5%    ~     (p=0.521 n=50+50)
Compiler          3.30s ± 3%        3.33s ± 4%  +0.76%  (p=0.003 n=50+49)
SSA               8.17s ± 4%        8.23s ± 3%  +0.66%  (p=0.030 n=50+49)
Flate             139ms ± 6%        138ms ± 8%    ~     (p=0.184 n=49+48)
GoParser          174ms ± 5%        172ms ± 6%    ~     (p=0.107 n=48+49)
Reflect           431ms ± 8%        420ms ± 5%  -2.57%  (p=0.000 n=50+46)
Tar               119ms ± 6%        118ms ± 7%  -0.95%  (p=0.033 n=50+49)
XML               236ms ± 4%        236ms ± 4%    ~     (p=0.935 n=50+48)
[Geo mean]        410ms             407ms       -0.67%

name        old alloc/op      new alloc/op      delta
Template         38.7MB ± 0%       38.6MB ± 0%  -0.29%  (p=0.008 n=5+5)
Unicode          29.8MB ± 0%       29.7MB ± 0%  -0.24%  (p=0.008 n=5+5)
GoTypes           113MB ± 0%        113MB ± 0%  -0.29%  (p=0.008 n=5+5)
Compiler          462MB ± 0%        462MB ± 0%  -0.12%  (p=0.008 n=5+5)
SSA              1.27GB ± 0%       1.27GB ± 0%  -0.05%  (p=0.008 n=5+5)
Flate            25.2MB ± 0%       25.1MB ± 0%  -0.37%  (p=0.008 n=5+5)
GoParser         31.7MB ± 0%       31.6MB ± 0%    ~     (p=0.056 n=5+5)
Reflect          77.5MB ± 0%       77.2MB ± 0%  -0.38%  (p=0.008 n=5+5)
Tar              26.4MB ± 0%       26.3MB ± 0%    ~     (p=0.151 n=5+5)
XML              41.9MB ± 0%       41.9MB ± 0%  -0.20%  (p=0.032 n=5+5)
[Geo mean]       74.5MB            74.3MB       -0.23%

name        old allocs/op     new allocs/op     delta
Template           378k ± 1%         377k ± 1%    ~     (p=0.690 n=5+5)
Unicode            321k ± 0%         322k ± 0%    ~     (p=0.595 n=5+5)
GoTypes           1.14M ± 0%        1.14M ± 0%    ~     (p=0.310 n=5+5)
Compiler          4.25M ± 0%        4.25M ± 0%    ~     (p=0.151 n=5+5)
SSA               9.84M ± 0%        9.84M ± 0%    ~     (p=0.841 n=5+5)
Flate              232k ± 1%         232k ± 0%    ~     (p=0.690 n=5+5)
GoParser           315k ± 1%         315k ± 1%    ~     (p=0.841 n=5+5)
Reflect            970k ± 0%         970k ± 0%    ~     (p=0.841 n=5+5)
Tar                248k ± 0%         248k ± 1%    ~     (p=0.841 n=5+5)
XML                389k ± 0%         389k ± 0%    ~     (p=1.000 n=5+5)
[Geo mean]         724k              724k       +0.01%

Updates #15756

Change-Id: I2646332e89f0444ca9d5a41d7172537d904ed636
Reviewed-on: https://go-review.googlesource.com/41050
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>