[mlir][transforms] Process RegionBranchOp with empty region

This PR adds process for RegionBranchOp with empty region, such as `scf.if`.

Author: CoTinker

mlir/lib/Transforms/RemoveDeadValues.cpp

@@ -375,6 +375,8 @@ static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
   // Mark live arguments in the regions of `regionBranchOp` in `liveArgs`.
   auto markLiveArgs = [&](DenseMap<Region *, BitVector> &liveArgs) {
     for (Region &region : regionBranchOp->getRegions()) {
+      if (region.empty())
+        continue;
       SmallVector<Value> arguments(region.front().getArguments());
       BitVector regionLiveArgs = markLives(arguments, nonLiveSet, la);
       liveArgs[&region] = regionLiveArgs;
@@ -420,6 +422,8 @@ static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
   auto markNonForwardedReturnValues =
       [&](DenseMap<Operation *, BitVector> &nonForwardedRets) {
         for (Region &region : regionBranchOp->getRegions()) {
+          if (region.empty())
+            continue;
           Operation *terminator = region.front().getTerminator();
           nonForwardedRets[terminator] =
               BitVector(terminator->getNumOperands(), true);
@@ -499,6 +503,8 @@ static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
         // Recompute `resultsToKeep` and `argsToKeep` based on
         // `terminatorOperandsToKeep`.
         for (Region &region : regionBranchOp->getRegions()) {
+          if (region.empty())
+            continue;
           Operation *terminator = region.front().getTerminator();
           for (const RegionSuccessor &successor : getSuccessors(&region)) {
             Region *successorRegion = successor.getSuccessor();
@@ -547,6 +553,8 @@ static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
 
           // Update the terminator operands that need to be kept.
           for (Region &region : regionBranchOp->getRegions()) {
+            if (region.empty())
+              continue;
             updateOperandsOrTerminatorOperandsToKeep(
                 terminatorOperandsToKeep[region.back().getTerminator()],
                 resultsToKeep, argsToKeep, &region);
@@ -611,8 +619,8 @@ static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
 
   // Do (2.a) and (2.b).
   for (Region &region : regionBranchOp->getRegions()) {
-    assert(!region.empty() && "expected a non-empty region in an op "
-                              "implementing `RegionBranchOpInterface`");
+    if (region.empty())
+      continue;
     BitVector argsToRemove = argsToKeep[&region].flip();
     cl.blocks.push_back({&region.front(), argsToRemove});
     collectNonLiveValues(nonLiveSet, region.front().getArguments(),
@@ -621,6 +629,8 @@ static void processRegionBranchOp(RegionBranchOpInterface regionBranchOp,
 
   // Do (2.c).
   for (Region &region : regionBranchOp->getRegions()) {
+    if (region.empty())
+      continue;
     Operation *terminator = region.front().getTerminator();
     cl.operands.push_back(
         {terminator, terminatorOperandsToKeep[terminator].flip()});