Unexpected output indexing arrays with array values


## *What* is wrong?
As part of a project that I'm trying to integrate GPU.js into to enhance performance, I've encountered some bizarre output when running a kernel. If indexing an array with another, the output appears to have been taken from the values of the _inner_ array. This is best explained with an example.

## *Where* does it happen?
See the following for a minimal reproducible example.

```
const gpu = new GPU({ mode: 'gpu' });
const arr1 = [0, 1, 3, 4];
const arr2 = [10, 11, 12, 13, 14];

const arrKernel = gpu.createKernel(function(arr1, arr2) {
return arr2[arr1[this.thread.x]];
}).setOutput([4]);
const result = arrKernel(arr1, arr2);

// Expected output: "10,11,13,14"
// jsfiddle output with mode: 'gpu': "0,1,4,0"
// jsfiddle output with mode: 'cpu': "10,11,13,14" (as expected)
console.log(result.toString());
```

1. Create two arrays
2. Use `this.thread.x` to index the first, and use the values of that to index the second
3. Observe unexpected output

I have a JS fiddle that can recreate this issue at [jsfiddle.net/TomWyllie/c5khtu9g/](https://jsfiddle.net/TomWyllie/c5khtu9g/). The source of the seemingly arbitrary output seems to be the values in the _inner_ array as I mentioned above. That is, if I had written `return arr1[arr1[this.thread.x]];` instead of `return arr2[arr1[this.thread.x]];` all would be as expected.

This was tested using:
* GPU.js 2.9.4
* GPU: NVIDIA GeForce GTX 1060 3GB
* Google Chrome Version 83.0.4103.97 (Official Build) (64-bit), but same issue in Firefox
* Windows 10

I couldn't replicate the issue on my phone (Samsung Galaxy S9) and haven't tried any other devices.

## *How* important is this (1-5)?
Probably a ~3~ 4, it's pretty annoying but is probably possible to work around in my use case. But it'd make things unnecessarily complicated and other people might spend time debugging not suspecting this has happened - it's a very strange result.

## Other Comments
I am very new to GPU.js (like, < 1 week) so am not 100% sure if this is a genuine bug, or if I have somehow ventured into unsupported territory, but given that the expected result is returned in `mode: cpu`, and also in `mode: gpu` on my phone in the JS Fiddle I thought it'd be worth flagging up. If I've missed something stupid please do let me know!

Finally, although not relevant to this issue in case anybody is wondering about the context, my use-case is traversing a matrix in diagonals, as only the diagonals can be computed in parallel due to the recursive nature of each entry. This is part of a string alignment algorithm. I am using (or trying to) `this.thread.x` to parameterise the position along each diagonal, the indices of which are computed outside the kernel.

Any comments or thoughts much appreciated! 😊

Start with the minimal reproducible createKernel example in the issue and run it in both gpu and cpu modes, comparing the result with the expected output. Investigate the array-indexing path used by the GPU kernel; done means gpu mode returns the same values as cpu mode for this example.

Scritto dal modello di indicizzazione a partire dal testo della issue.