[llvm] [AMDGPU] Reland: Do not use original PHIs in coercion chains (PR #98985)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

https://github.com/llvm/llvm-project/commit/6e68b75e6634e9809e2af5d68c87a4eb4282217e

Thanks for review again. I agree that this approach is likely not the best, and will find a better way to deal with broken chains.

https://github.com/llvm/llvm-project/pull/98985