[mlir][llvm] Ordered traversal in LLVM IR import.

The revision performs a topological sort of the blocks to
ensure the operations are processed in dominance order.
After the change, we do not need to introduce dummy
instructions if an operand has not yet been processed.
Additionally, the revision also moves and simplifies the
control-flow related tests to a separate test file.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D136230