[clang] [flang] [mlir] [mlir][Interfaces][NFC] Simplify and align `RegionSuccessor` design / API (PR #174945)

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mtsamis wrote:

Thanks for the additional context.

I cannot comment on how easy it'll be to backport, but the reason I brought this up is because the version in the release candidate is in a in-between state of these improvements. The lack of a replacement for `RegionSuccessor()` (which as far as I understand is not really equivalent to `RegionSuccessor(getOperation(), getOperation()->getResults())` has been slightly confusing in comparison to `RegionSuccessor::parent()` which is straightforward.



https://github.com/llvm/llvm-project/pull/174945