[CI][lit] Enable roundtrip tests in CI (#7377)

This PR adds a round trip tests In CI. This could slightly increase the test time but it's
ignorable compared to build time. 

cf. a6d09d4b1a

Close https://github.com/llvm/circt/issues/5272

.github/workflows/buildAndTest.yml

@@ -222,7 +222,7 @@ jobs:
             -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
             -DLLVM_LIT_ARGS="-v" \
             -DCIRCT_SLANG_FRONTEND_ENABLED=ON
-          ninja check-circt check-circt-unit -j$(nproc)
+          CIRCT_OPT_CHECK_IR_ROUNDTRIP=1 ninja check-circt check-circt-unit -j$(nproc)
           ninja circt-doc
 
       # --------

test/lit.cfg.py

@@ -51,6 +51,8 @@ config.test_exec_root = os.path.join(config.circt_obj_root, 'test')
 
 # Tweak the PATH to include the tools dir.
 llvm_config.with_environment('PATH', config.llvm_tools_dir, append_path=True)
+llvm_config.with_environment('PATH', config.mlir_tools_dir, append_path=True)
+llvm_config.with_environment('PATH', config.circt_tools_dir, append_path=True)
 
 tool_dirs = [
     config.circt_tools_dir, config.mlir_tools_dir, config.llvm_tools_dir
@@ -58,10 +60,18 @@ tool_dirs = [
 tools = [
     'arcilator', 'circt-as', 'circt-capi-ir-test', 'circt-capi-om-test',
     'circt-capi-firrtl-test', 'circt-capi-firtool-test', 'circt-dis',
-    'circt-opt', 'circt-reduce', 'circt-translate', 'firtool', 'hlstool',
-    'om-linker', 'ibistool'
+    'circt-reduce', 'circt-translate', 'firtool', 'hlstool', 'om-linker',
+    'ibistool'
 ]
 
+if "CIRCT_OPT_CHECK_IR_ROUNDTRIP" in os.environ:
+  tools.extend([
+      ToolSubst("circt-opt", "circt-opt --verify-roundtrip",
+                unresolved="fatal"),
+  ])
+else:
+  tools.extend(["circt-opt"])
+
 # Enable Verilator if it has been detected.
 if config.verilator_path != "":
   tool_dirs.append(os.path.dirname(config.verilator_path))