[llvm] [AMDGPU][NPM] Add target-specific register allocation options (PR #178889)

[Teja Alaghari via llvm-commits]

TejaX-Alaghari wrote:

> This LGTM in general, but I have a question/suggestion: Could we teach the legacy infra about the RegAllocType enum? I'm wondering if instead of having a set of options for the legacy PM and one for the npm, we could have only the new options (ideally with the old names) plus some glue code that teaches the legacy PM what we're trying to do based on the parsed enum value.

Thanks for the suggestion @rovka. I agree with this being a good idea in principle.

But I'd prefer to keep them separate for now in this PR, as the type systems are different in the Legacy and NPM (RegisterRegAlloc function pointer vs RegAllocType enum) making the glue code non-trivial.

And once legacy PM is deprecated, we could add the translation layer as a follow-up to consolidate options.
Happy to revisit if this is a blocker.

https://github.com/llvm/llvm-project/pull/178889