[mlir][Interfaces] Split successor inputs from region successor (#175815) This commit simplifies the design of the `RegionBranchOpInterface`. The property of being a successor input is now independent of the region branch point. There is a new API for querying successor inputs: `RegionBranchOpInterface::getSuccessorInputs(RegionSuccessor)`. Note that this function does **not** take a `RegionBranchPoint` as parameter. The `RegionSuccessor` API is now also simpler: it no longer stores successor inputs. A region successor is simply `Region *`, wrapped around a convenience API. Note: This commit is mostly mechanical. Analyses / transformations that build on top of the `RegionBranchOpInterface` (e.g., `visitNonControlFlowArguments` API) can likely be simplified in follow-up commits. Note for LLVM integration: Split `RegionBranchOpInterface::getSuccessorRegion` implementations into two functions: `getSuccessorRegion` and `getSuccessorInputs. (There are many examples in this commit.) RFC: https://discourse.llvm.org/t/rfc-simplify-regionbranchopinterface-separate-successor-inputs-from-region-successor/89420/7 GitOrigin-RevId: f76433761a55d47b31dd65150a0a5b6ae923c673
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