Add "not" optimization.

[Kailo97]

if (!func())

Gives

not
jzer n

It has to replace to

jnz n

[asherkin]

This lgtm (being a lift from upstream), but I'd like @Fyren or @dvander to give this a once over before merging as they're more familiar with this beast and our changes to it.

[Kailo97]

About this case (probably not generatable)

eq.c.pri 0
not
jzer n1

all still worse

if (!(g_int[0] == 0))

gives

const.pri 2228
load.i
zero.alt
eq
not
jzer 3292

[dvander]

This seems reasonable.