[llvm] [AMDGPU] Add option to preinflate to AVGPR (PR #147413)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

A few things:
- Can this go into AMDGPUPrepareAGPRAlloc instead of GCNPreRAOptimizations?
- It should only be done if the occupancy target is already down to 1 (i.e. not all AGPRs are reserved)
- We should just do this by default (i.e. remove the flag or flip the default)

https://github.com/llvm/llvm-project/pull/147413