See also: https://mlir.llvm.org/docs/LangRef/#blocks

"Context: The “block argument” representation eliminates a number of special cases from the IR compared to traditional “PHI nodes are operations” SSA IRs (like LLVM). For example, the parallel copy semantics of SSA is immediately apparent, and function arguments are no longer a special case: they become arguments to the entry block [ more rationale ]."

Block Arguments vs PHI nodes: https://mlir.llvm.org/docs/Rationale/#block-arguments-vs-phi...