[mlir] Fix verifier of `RegionBranchOpInterface`

The verifier incorrectly passed the region number of the predecessor region instead of the successor region to `getSuccessorOperands`. This went unnoticed since all upstream `RegionBranchTerminatorOpInterface` implementations did not make use of the `index` parameter.
Adding an assert to e.g. `scf.condition` to make sure the index is valid or adding a region terminator that passes different operands to different successors immediately causes the verifier to fail as it suddenly gets incorrect types.

This patch fixes the implementation to correctly pass the successor region index.

Differential Revision: https://reviews.llvm.org/D157507