[naga]Naga does not generate for loops

[b0nes164]

Currently, naga exclusively generates while loops when translating code for downstream compilers. This:

• Breaks compilation of some shaders on FXC because loops can't be unrolled so arrays cannot be dynamically indexed.
• Prior to [naga msl-out] Defeat the MSL compiler's infinite loop analysis. #6285, potentially caused UB on metal backend.
• Causes suboptimal code generation on downstream compilers, again because loops cannot be unrolled. See for loops get translated into complex control-flow that confuses downstream compilers #4499.

To measure the impact on performance this is causing, I implemented my prefix sum demo in wgpu + naga and in dawn + tint, compiling the shaders from the exact same wgsl code. The results on an Apple M1 Pro(8 + 2 version):

I can't be certain that the loop unrolling is 100% of the performance discrepancy but here's what I can say:

• It's not occupancy related. That is to say, it doesn't seem like inefficient translation into metal is eating up register counts. Using a hack technique, I obtained identical occupancy estimations from all implementations.

• It's not an artifact of the data collection. Other metrics gathered during the tests show that threadblocks are indeed waiting longer on wgpu+naga:

//wgpu+naga 22.0 Decoupled Fallback
Estimated Occupancy across GPU: 80
Thread Blocks Launched: 8192
Average Total Spins per Pass: 10181.346
Average Fallbacks Initiated per Pass: 1867.042
Average Successful Fallback Insertions per Pass: 2.922

//wgpu+naga 23.0 Decoupled Fallback
Estimated Occupancy across GPU: 80
Thread Blocks Launched: 8192
Average Total Spins per Pass: 5527.18
Average Fallbacks Initiated per Pass: 1026.484
Average Successful Fallback Insertions per Pass: 0.082

//dawn+tint Decoupled Fallback
Estimated Occupancy across GPU: 80
Thread Blocks Launched:  8192
Average Total Spins per Pass: 3625.62
Average Fallbacks Initiated per Pass:  362.902
Average Successful Fallback Insertions per Pass: 0.116

The uptick in performance in between wgpu 22.0 and 23.0 suggests that #4972 was also causing slowdowns. However, the fix may be introducing new problems that are also contributing to the performance discrepancy see #6518.

[b0nes164]

The first thing I forgot to mention is that also data gathered here was already on unchecked shaders. If this was purely performance degradation caused caused by 6285, I would expect a performance decrease from 22.0 -> 23.0, not the increase we are seeing here.

I also did not put forward what a solution would be. I don't think all for loops need be translated, though that would be nice. Instead, a guarantee to translate for loops that meet certain conditions--compiler visible constant on the conditional, no complicated control flow--would be enough to solve these issues.