[Calyx] [NFC] Use stricter Block existence, not Region existence for IfOp.

include/circt/Dialect/Calyx/CalyxControl.td

@@ -70,17 +70,27 @@ def IfOp : CalyxContainer<"if", [
   let verifier = "return ::verify$cppClass(*this);";
   let hasCanonicalizeMethod = true;
   let extraClassDeclaration = [{
-    /// Checks whether the `then` region exists.
-    bool thenRegionExists() { return !getOperation()->getRegion(0).empty(); }
-    /// Checks whether the `else` region exists.
-    bool elseRegionExists() { return !getOperation()->getRegion(1).empty(); }
+    /// Checks whether the `then` body exists.
+    bool thenBodyExists() {
+      Region* region = &getOperation()->getRegion(0);
+      if (region == nullptr)
+        return false;
+      return !region->empty();
+    }
+    /// Checks whether the `else` body exists.
+    bool elseBodyExists() {
+      Region* region = &getOperation()->getRegion(1);
+      if (region == nullptr)
+        return false;
+      return !region->empty();
+    }
     /// Gets the single basic block representing the `then` region.
     Block *getThenBody() {
       return &getOperation()->getRegion(0).front();
     }
     /// Gets the single basic block representing the `else` region.
     Block *getElseBody() {
-      assert(elseRegionExists() && "Else region does not exist.");
+      assert(elseBodyExists() && "Else region does not exist.");
       return &getOperation()->getRegion(1).front();
     }
   }];

lib/Dialect/Calyx/CalyxOps.cpp

@@ -1377,7 +1377,7 @@ static LogicalResult verifyIfOp(IfOp ifOp) {
   if (ifOp.getThenBody()->empty())
     return ifOp.emitError() << "empty 'then' region.";
 
-  if (ifOp.elseRegionExists() && ifOp.getElseBody()->empty())
+  if (ifOp.elseBodyExists() && ifOp.getElseBody()->empty())
     return ifOp.emitError() << "empty 'else' region.";
 
   Optional<StringRef> optGroupName = ifOp.groupName();
@@ -1428,7 +1428,7 @@ static EnableOp getLastEnableOp(SeqOp parent) {
 static LogicalResult eliminateCommonTailEnable(IfOp ifOp,
                                                PatternRewriter &rewriter) {
   // Check if the branches exist.
-  if (!ifOp.thenRegionExists() || !ifOp.elseRegionExists())
+  if (!ifOp.thenBodyExists() || !ifOp.elseBodyExists())
     return failure();
 
   auto &thenOpStructureOp = ifOp.getThenBody()->front();

lib/Dialect/Calyx/Export/CalyxEmitter.cpp

@@ -406,7 +406,7 @@ private:
               os << " with " << groupName.getValue();
 
             emitCalyxBody([&]() { emitCalyxControl(op.getThenBody()); });
-            if (op.elseRegionExists())
+            if (op.elseBodyExists())
               emitCalyxSection("else",
                                [&]() { emitCalyxControl(op.getElseBody()); });
           })