[llvm] [mlir] [Flang][OpenMP][OpenMPIRBuilder] Implement module scope declare target use rewrite mechanism (PR #212920)

[Sairudra More via llvm-commits]

https://github.com/Saieiei requested changes to this pull request.

Thanks for the patch. I found one correctness issue in the new module-scope replacement loop involving direct PHI users, which can produce invalid LLVM IR. Please address the inline comment and add regression coverage.

https://github.com/llvm/llvm-project/pull/212920