[llvm] [AMDGPU] Add backward compatibility layer for kernarg preloading (PR #119167)

[Matt Arsenault via llvm-commits]

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@@ -161,6 +162,8 @@ struct AMDGPUFunctionArgInfo {
 
   // Map the index of preloaded kernel arguments to its descriptor.
   SmallDenseMap<int, KernArgPreloadDescriptor> PreloadKernArgs{};
+  // The first user SGPR allocated for kernarg preloading.
+  Register FirstKernArgPreloadReg = AMDGPU::NoRegister;
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arsenm wrote:

```suggestion
  Register FirstKernArgPreloadReg;
```

This is already the default and we're trying to get rid of the unsigned -> Register conversion 

https://github.com/llvm/llvm-project/pull/119167