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

[Matthias Springer via cfe-commits]

matthias-springer wrote:

I believe `RegionSuccessor(results)` and `RegionSuccessor(getOperation(), results)` are equivalent. The extra `Operation *` parameter was added in preparation of "early exit from region", but that hasn't been merged yet. Maybe we can just mention in the release notes how to migrate the code? (I.e., just add `getOperation()` as first parameter.) This part of the code base has been edited heavily and back-porting could be a pain. (However, if you find somebody who is willing to dig through the code, it could make for a nicer user experience.)

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