[llvm] [DebugInfo][RemoveDIs] Assert if we mix PHIs and debug-info (PR #84054)

[Orlando Cazalet-Hyams via llvm-commits]

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

Makes sense, and should help people track down errant PHI insertions more quickly. SGTM.

https://github.com/llvm/llvm-project/pull/84054