Add IfOp helpers. (#1870)

Add helper methods for the IfOp. The IfOp inherits from CalyxContainer, which has the SingleBlock trait, 
so it should be guaranteed that each region has a single block. I've renamed the getters to remain in line 
with the getBody helper for other Calyx containers.

include/circt/Dialect/Calyx/CalyxControl.td

@@ -69,11 +69,16 @@ def IfOp : CalyxContainer<"if", [
   let assemblyFormat = "$cond (`with` $groupName^)? $thenRegion (`else` $elseRegion^)? attr-dict";
   let verifier = "return ::verify$cppClass(*this);";
   let extraClassDeclaration = [{
-    Block *getThenRegion() {
+    /// 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(); }
+    /// Gets the single basic block representing the `then` region.
+    Block *getThenBody() {
       return &getOperation()->getRegion(0).front();
     }
-    bool elseRegionExists() { return !getOperation()->getRegion(1).empty(); }
-    Block *getElseRegion() {
+    /// Gets the single basic block representing the `else` region.
+    Block *getElseBody() {
       assert(elseRegionExists() && "Else region does not exist.");
       return &getOperation()->getRegion(1).front();
     }

lib/Conversion/SCFToCalyx/SCFToCalyx.cpp

@@ -1278,7 +1278,7 @@ struct EliminateEmptyOpPattern : mlir::OpRewritePattern<TOp> {
 
   LogicalResult matchAndRewrite(TOp op,
                                 PatternRewriter &rewriter) const override {
-    if (op.getRegion().empty() || op.getRegion().front().empty()) {
+    if (op.getBody()->empty()) {
       rewriter.eraseOp(op);
       return success();
     }
@@ -1293,7 +1293,7 @@ struct EliminateEmptyOpPattern<calyx::IfOp>
 
   LogicalResult matchAndRewrite(calyx::IfOp op,
                                 PatternRewriter &rewriter) const override {
-    if (op.thenRegion().empty() || op.thenRegion().front().empty()) {
+    if (!op.thenRegionExists()) {
       rewriter.eraseOp(op);
       return success();
     }
@@ -1432,13 +1432,11 @@ struct CommonIfTailEnablePattern : mlir::OpRewritePattern<calyx::IfOp> {
     /// Check if there's anything in the branches; if not,
     /// EliminateEmptyOpPattern will eliminate a potentially
     /// empty/invalid if statement.
-    if (ifOp.thenRegion().empty() || ifOp.thenRegion().front().empty())
-      return failure();
-    if (ifOp.elseRegion().empty() || ifOp.elseRegion().front().empty())
+    if (!ifOp.thenRegionExists() || !ifOp.elseRegionExists())
       return failure();
 
-    auto &thenOpStructureOp = ifOp.thenRegion().front().front();
-    auto &elseOpStructureOp = ifOp.elseRegion().front().front();
+    auto &thenOpStructureOp = ifOp.getThenBody()->front();
+    auto &elseOpStructureOp = ifOp.getElseBody()->front();
     if (isa<calyx::ParOp>(thenOpStructureOp) ||
         isa<calyx::ParOp>(elseOpStructureOp))
       return failure();

lib/Dialect/Calyx/Export/CalyxEmitter.cpp

@@ -405,10 +405,10 @@ struct Emitter {
             if (auto groupName = op.groupName(); groupName.hasValue())
               os << " with " << groupName.getValue();
 
-            emitCalyxBody([&]() { emitCalyxControl(op.getThenRegion()); });
+            emitCalyxBody([&]() { emitCalyxControl(op.getThenBody()); });
             if (op.elseRegionExists())
               emitCalyxSection("else",
-                               [&]() { emitCalyxControl(op.getElseRegion()); });
+                               [&]() { emitCalyxControl(op.getElseBody()); });
           })
           .Case<EnableOp>([&](auto op) { emitEnable(op); })
           .Default([&](auto op) {