[llvm] [AMDGPU] Simplify and improve codegen for llvm.amdgcn.set.inactive (PR #107889)

[Jay Foad via llvm-commits]

jayfoad wrote:

Please see commit messages for individual commits. They're intended to be reviewed separately, but committed together to avoid code quality regressions.

https://github.com/llvm/llvm-project/pull/107889