[acorn-optimizer] unsignPointers can cause problems optional second argument `subarray`

The `unsignPointers` pass will currently add `>>> 0` to all arguments to `\.subarray`.

However, if the value being passed is `undefined` this changes its meaning.

The case I found was in `library_glemu.js` which does something like this: `HEAP16.subarray(ptr, end ? end : undefined)`.

In the case when `end` is undefined this ends up being `end >>> 0` which is 0, and which is very different to `undefined` when passed as this argument.