Implement it through a partial order datastructure, which
keeps the relations between SSA values in a forest of DAGs
and is able to discover contradictions.

In make.bash, this patch is able to prove hundreds of conditions
which were not proved before.

Compilebench:

name       old time/op       new time/op       delta
Template         371ms ± 2%        368ms ± 1%    ~     (p=0.222 n=5+5)
Unicode          203ms ± 6%        199ms ± 3%    ~     (p=0.421 n=5+5)
GoTypes          1.17s ± 4%        1.18s ± 1%    ~     (p=0.151 n=5+5)
Compiler         5.54s ± 2%        5.59s ± 1%    ~     (p=0.548 n=5+5)
SSA              12.9s ± 2%        13.2s ± 1%  +2.96%  (p=0.032 n=5+5)
Flate            245ms ± 2%        247ms ± 3%    ~     (p=0.690 n=5+5)
GoParser         302ms ± 6%        302ms ± 4%    ~     (p=0.548 n=5+5)
Reflect          764ms ± 4%        773ms ± 3%    ~     (p=0.095 n=5+5)
Tar              354ms ± 6%        361ms ± 3%    ~     (p=0.222 n=5+5)
XML              434ms ± 3%        429ms ± 1%    ~     (p=0.421 n=5+5)
StdCmd           22.6s ± 1%        22.9s ± 1%  +1.40%  (p=0.032 n=5+5)

name       old user-time/op  new user-time/op  delta
Template         436ms ± 8%        426ms ± 5%    ~     (p=0.579 n=5+5)
Unicode          219ms ±15%        219ms ±12%    ~     (p=1.000 n=5+5)
GoTypes          1.47s ± 6%        1.53s ± 6%    ~     (p=0.222 n=5+5)
Compiler         7.26s ± 4%        7.40s ± 2%    ~     (p=0.389 n=5+5)
SSA              17.7s ± 4%        18.5s ± 4%  +4.13%  (p=0.032 n=5+5)
Flate            257ms ± 5%        268ms ± 9%    ~     (p=0.333 n=5+5)
GoParser         354ms ± 6%        348ms ± 6%    ~     (p=0.913 n=5+5)
Reflect          904ms ± 2%        944ms ± 4%    ~     (p=0.056 n=5+5)
Tar              398ms ±11%        430ms ± 7%    ~     (p=0.079 n=5+5)
XML              501ms ± 7%        489ms ± 5%    ~     (p=0.444 n=5+5)

name       old text-bytes    new text-bytes    delta
HelloSize        670kB ± 0%        670kB ± 0%  +0.00%  (p=0.008 n=5+5)
CmdGoSize       7.22MB ± 0%       7.21MB ± 0%  -0.07%  (p=0.008 n=5+5)

name       old data-bytes    new data-bytes    delta
HelloSize       9.88kB ± 0%       9.88kB ± 0%    ~     (all equal)
CmdGoSize        248kB ± 0%        248kB ± 0%  -0.06%  (p=0.008 n=5+5)

name       old bss-bytes     new bss-bytes     delta
HelloSize        125kB ± 0%        125kB ± 0%    ~     (all equal)
CmdGoSize        145kB ± 0%        144kB ± 0%  -0.20%  (p=0.008 n=5+5)

name       old exe-bytes     new exe-bytes     delta
HelloSize       1.43MB ± 0%       1.43MB ± 0%    ~     (all equal)
CmdGoSize       14.5MB ± 0%       14.5MB ± 0%  -0.06%  (p=0.008 n=5+5)

Fixes #19714
Updates #20393

Change-Id: Ia090f5b5dc1bcd274ba8a39b233c1e1ace1b330e
Reviewed-on: https://go-review.googlesource.com/100277
Run-TryBot: Giovanni Bajo <rasky@develer.com>
Reviewed-by: David Chase <drchase@google.com>