* Decreases the generated code slightly.
* Similar to phiopt pass from gcc, except it only handles
booleans. Handling Eq/Neq had no impact on the generated code.

name       old time/op     new time/op     delta
Template       453ms ± 4%      451ms ± 4%    ~           (p=0.468 n=24+24)
GoTypes        1.55s ± 1%      1.55s ± 2%    ~           (p=0.287 n=24+25)
Compiler       6.53s ± 2%      6.56s ± 1%  +0.46%        (p=0.050 n=23+23)
MakeBash       45.8s ± 2%      45.7s ± 2%    ~           (p=0.866 n=24+25)

name       old text-bytes  new text-bytes  delta
HelloSize       676k ± 0%       676k ± 0%    ~     (all samples are equal)
CmdGoSize      8.07M ± 0%      8.07M ± 0%  -0.03%        (p=0.000 n=25+25)

Change-Id: Ia62477b7554127958a14cb27f85849b095d63663
Reviewed-on: https://go-review.googlesource.com/20090Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>