[mlir] Remove assert from RegionBranchOpInterface verifier

This assert is erroneous because an op implementing
`RegionBranchOpInterface` can have variadic regions and in some cases
have zero regions, in which case the only possible control flow is
branching from the parent op to itself.

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D136052