[flang] [llvm] [mlir] [flang][MLIR][OpenMP] make reduction by-ref toggled per variable (PR #92244)

[Tom Eccles via llvm-commits]

tblah wrote:

> Looks mostly OK to me. Have some minor comments inline.
> 
> Would adding the `byref` attribute to the Reduction Declaration Operation work?

Thanks for taking a look. Yes I think adding it to the ReductionDeclareOp would also work

https://github.com/llvm/llvm-project/pull/92244