[llvm] [AMDGPU] Improve amdgcn_kill verification (PR #208220)

[Robert Imschweiler via llvm-commits]

ro-i wrote:

yeah, makes sense. This also led me to relax the `unreachable` verification, as well, although that wasn't my initial intention (but ig it makes more sense)

https://github.com/llvm/llvm-project/pull/208220