[llvm] [AMDGPU][NPM] Support -regalloc-npm options (PR #129035)

[Akshat Oke via llvm-commits]

optimisan wrote:

> I would prefer generic regalloc pipeline tunning options, X86 backend also has the same feature request, but it is related to generic builder design. I'm fine with this change if AMDGPU maintainers approve it.

Is there any generic design proposal for regalloc pipeline? I remember something like `-regalloc-npm="greedy<sgpr>,fast<wwm>"` was to be done. This means the codegen pipeline will call a separate regalloc pipeline to provide the appropriate RAPass to be inserted in the codegen pipeline.

We could wait till the final design of the TargetPassBuilder is finalized, but here is my understanding of this:

Adding regalloc passes would be done with a generic method `addRegAllocPass()` like so:
```cpp
void CodeGenPassBuilder::addRegAssignmentAndRewriteOptimized() {
             addPass(...);
             ...
             addRegAllocPass<RAGreedyPass>();
             ...
             addRegAllocPass<RAGreedyPass>();
             ...
             addRegAllocPass<RAGreedyPass>();
             ...
}
```

Targets have to specify which filters (and options) to use for which phase (that are now numbered)
The generic implementation would be:

```cpp
void CodeGenPassBuilder::addRegAllocPass<SelectedRAPassType>( &MFPM ) {
     static unsigned phase = 1;
     if(cliRAPipeline)
            // add whatever is in -regalloc-npm option
     else
         MFPM.addPass(SelectedRAPassType(getTargetRAPhaseFilter(phase)));
     phase++;
}
```

https://github.com/llvm/llvm-project/pull/129035