[naga] Infinite loop mitigation needs to count down, not up

[jimblandy]

Google Chrome's WebGPU team tells us that certain Intel drivers hang if the infinite loop mitigation described in #6572 counts up from zero. However, counting down from u32::MAX is okay. 🤷‍♂

Naga should adjust its workarounds to count downwards instead of upwards.

The "full employment for GPU programmers" plan is going swimmingly!

[cwfitzgerald]

This is why gpu programmers have trust issues

[jamienicol]