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

[Jeremy Morse via llvm-commits]

jmorse wrote:

Testing: this compiles stage2reldeb clang successfully. I've also today merged all our modifications of PHI-creation sites to always use iterators, so it Shouldn't (TM) fire at all.

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