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

[Matt Arsenault via llvm-commits]

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

LGTM but I think this should try to use ValueMap and/or WeakTrackingVH and replacements. This approach seems more complicated than necessary to keep values updated 

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