[FIRRTL][CheckCombLoops] Switch to the new pass checkCombLoops instea…

…d of checkCombCycles (#4513)

Switch to the new CheckCombLoops pass and add a flag
 `--use-old-check-comb-cycles` to switch to the old `CheckCombCycles` pass.

tools/firtool/firtool.cpp

@@ -304,6 +304,11 @@ static cl::opt<bool>
                            cl::desc("Disable the CheckCombCycles pass"),
                            cl::init(false), cl::Hidden, cl::cat(mainCategory));
 
+static cl::opt<bool> useOldCheckCombCycles(
+    "use-old-check-comb-cycles",
+    cl::desc("Use old CheckCombCycles pass, that does not support aggregates"),
+    cl::init(false), cl::Hidden, cl::cat(mainCategory));
+
 static cl::opt<bool> disableIMDCE("disable-imdce",
                                   cl::desc("Disable the IMDCE pass"),
                                   cl::init(false), cl::Hidden,
@@ -692,14 +697,16 @@ static LogicalResult processBuffer(
         firrtl::createRandomizeRegisterInitPass());
 
   if (!disableCheckCombCycles) {
-    // TODO: Currently CheckCombCyles pass doesn't support aggregates so skip
-    // the pass for now.
-    if (preserveAggregate == firrtl::PreserveAggregate::None)
-      pm.nest<firrtl::CircuitOp>().addPass(firrtl::createCheckCombCyclesPass());
-    else
-      emitWarning(module->getLoc())
-          << "CheckCombCyclesPass doens't support aggregate "
-             "values yet so it is skipped\n";
+    if (useOldCheckCombCycles) {
+      if (preserveAggregate == firrtl::PreserveAggregate::None)
+        pm.nest<firrtl::CircuitOp>().addPass(
+            firrtl::createCheckCombCyclesPass());
+      else
+        emitWarning(module->getLoc())
+            << "CheckCombCyclesPass doens't support aggregate "
+               "values yet so it is skipped\n";
+    } else
+      pm.nest<firrtl::CircuitOp>().addPass(firrtl::createCheckCombLoopsPass());
   }
 
   // If we parsed a FIRRTL file and have optimizations enabled, clean it up.