[llvm] [AMDGPU] Implement wwm-register allocation (PR #86012)

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

Depends on https://github.com/llvm/llvm-project/pull/86006 https://github.com/llvm/llvm-project/pull/86007 https://github.com/llvm/llvm-project/pull/86008 (all pre-patches included in the commits list)

https://github.com/llvm/llvm-project/pull/86012