[mlir] Support getSuccessorInputs from parent op

Ops that implement `RegionBranchOpInterface` are allowed to indicate that they can branch back to themselves in `getSuccessorRegions`, but there is no API that allows them to specify the forwarded operands. This patch enables that by changing `getSuccessorEntryOperands` to accept `None`.

Fixes #54928

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D127239