bug: preserve for-loop / function parameter
This cart runs fine in pico8; it prints 9 numbers with some colors:
```lua
-- test2.lua
function _draw()
 cls()
 for --[[preserve]] n=1,3 do
  for i=1,3 do
   print(i+1,n+1)
  end
 end
end
```
(btw [be careful](https://www.lexaloffle.com/bbs/?tid=54395) with that annotation -- the spaces are required)

But minifying breaks things -- the minified cart is upset because `o+1` is nil arithmetic:
`shrinko8 --minify -f lua --no-minify-lines --no-minify-spaces test2.lua out.lua`
```lua
-- out.lua
function _draw()
 cls()
 for  n=1,3 do
  for n=1,3 do
   print(n+1,o+1)
  end
 end
end
```

---

another similarly bugged example:
```sh
> echo 'function foo(--[[preserve]] x) return x+1 end' | shrinko8 --minify -f lua - -
function n(x)return n+1end
```

(I would expect `function n(x)return x+1end` instead)

---

I initially tried to preserve these vars from the commandline (`--preserve 'n'`) but it didn't work. However, local vars declared like `local n=3` were correctly preserved. Maybe this is related?

My shrinko8 is the commit tagged `v1.1.2` ("avoid bxBX...")