[MLIR][LLVM] Fixed `topologicalSort()` to iterative version

Instead of recursive helper method `topologicalSortImpl()`,
sort's implementation is moved to `topologicalSort()` function's
body directly. `llvm::ReversePostOrderTraversal` is used to create
a traversal of blocks in reverse post order.

Reviewed By: kiranchandramohan, rriddle

Differential Revision: https://reviews.llvm.org/D88544