[clang] [llvm] [mlir] [OpenMP] Migrate GPU Reductions CodeGen from Clang to OMPIRBuilder (PR #80343)

[Johannes Doerfert via cfe-commits]

https://github.com/jdoerfert commented:

So, just to double check. On the Clang side, all you did is move the codegen and adapt it to work in the OMPIRBuilder, right? Expected outcome is the same, modulo the two small changes.

https://github.com/llvm/llvm-project/pull/80343