Print the // ---- separator between modules when using -split-input-file with mlir-opt

This allows to pipe sequences of `mlir-opt -split-input-file | mlir-opt -split-input-file`.

Depends On D117750

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D117756

Author: joker-eph

mlir/lib/Support/MlirOptMain.cpp

@@ -158,10 +158,12 @@ LogicalResult mlir::MlirOptMain(raw_ostream &outputStream,
     return splitAndProcessBuffer(
         std::move(buffer),
         [&](std::unique_ptr<MemoryBuffer> chunkBuffer, raw_ostream &os) {
-          return processBuffer(os, std::move(chunkBuffer), verifyDiagnostics,
-                               verifyPasses, allowUnregisteredDialects,
-                               preloadDialectsInContext, passManagerSetupFn,
-                               registry, threadPool);
+          LogicalResult result = processBuffer(
+              os, std::move(chunkBuffer), verifyDiagnostics, verifyPasses,
+              allowUnregisteredDialects, preloadDialectsInContext,
+              passManagerSetupFn, registry, threadPool);
+          os << "// -----\n";
+          return result;
         },
         outputStream);
 

mlir/test/Dialect/OpenACC/ops.mlir

@@ -1,8 +1,8 @@
 // RUN: mlir-opt -split-input-file %s | FileCheck %s
 // Verify the printed output can be parsed.
-// RUN: mlir-opt -split-input-file %s | mlir-opt -allow-unregistered-dialect  | FileCheck %s
+// RUN: mlir-opt -split-input-file %s | mlir-opt  -split-input-file  | FileCheck %s
 // Verify the generic form can be parsed.
-// RUN: mlir-opt -split-input-file -mlir-print-op-generic %s | mlir-opt -allow-unregistered-dialect | FileCheck %s
+// RUN: mlir-opt -split-input-file -mlir-print-op-generic %s | mlir-opt -split-input-file | FileCheck %s
 
 func @compute1(%A: memref<10x10xf32>, %B: memref<10x10xf32>, %C: memref<10x10xf32>) -> memref<10x10xf32> {
   %c0 = arith.constant 0 : index