[llvm] AMDGPU: Mark the SCC def dead when expanding CF pseudos (PR #226249)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Although in practice all these SCC defs should be dead (shouldn't they?) so it barely seems worth the effort of copying the dead-ness, you could just mark them dead unconditionally.

Yes, but that's not a guaranteed property 

https://github.com/llvm/llvm-project/pull/226249