[clang] [llvm] [AMDGPU][clang][CodeGen][opt] Add late-resolved feature identifying predicates (PR #134016)

[Jon Chesterfield via cfe-commits]

https://github.com/JonChesterfield requested changes to this pull request.

Request using intrinsics to represent this, not magic global variables, on the grounds that the magic globals hack should not have been done in the first place and definitely didn't need to live for a decade in rocm device libs.

Given how long it has existed there, I do not have confidence it would be fixed after commit in upstream. Further, I think someone else fixing it upstream would be opposed by the rocm developers on grounds of divergence from their fork.

https://github.com/llvm/llvm-project/pull/134016