Declare variable 'ffi' in global scope

The other part in the codebase where I saw *Inf* was being used, it was previously defined as ```local Inf = 1/0``` https://github.com/Igalia/pflua/blob/master/src/pf/optimize.lua#L402.

If I do 1/0 in Luajit REPL I got:
```
> = 1/0
inf
> 
```

It seems neither *Inf* or *inf* are symbols defined in the global scope. It's not that Snabbswitch removes them, actually it tries to find them in the global scope and since cannot find them, returns an error. In our case, this undefined *Inf* variable is the same as *nil*.


```
> = _G.inf
nil
```

I looked up a bit into this and it seems the symbol *inf* returned by the REPL is the same as ```math.huge```. From http://lua-users.org/wiki/MathLibraryTutorial:

```
math.huge

math.huge is a constant. It represents +infinity.

> = math.huge
inf
> = math.huge / 2
inf
```

So we can either replace *Inf* for ```math.huge``` or defined it before. Personally, I like the idea of having an *Inf* constant, it express intention better than ```math.huge```.