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

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

LGTM preferably with a rebase on #226406.

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.

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