"Variable $i might not be defined." in for loop using comma operator (binary search)

# Bug report

While implementing a binary search, we used a comma operator within a for loop and use the `$i` variable outside the loop too, and it is in practice always defined.

### Code snippet that reproduces the problem

https://phpstan.org/r/85c66c6f-a6d4-4084-8815-07d3f46b099d
https://3v4l.org/LVt1P

### Expected output

No errors
