[MLIR] Add primitive builders for scf.if

[jpienaar]

Nit: commit message. The rest looks fine (I forget if we allowed default values in OpBuilder ... )

[frgossen]

Thanks! Default values are possible but then it collides with the other builder

mlir/include/mlir/Dialect/SCF/IR/SCFOps.td

def IfOp : SCF_Op<"if", [DeclareOpInterfaceMethods<RegionBranchOpInterface, [
   let arguments = (ins I1:$condition);
   let results = (outs Variadic<AnyType>:$results);
   let regions = (region SizedRegion<1>:$thenRegion,
                         MaxSizedRegion<1>:$elseRegion);
 
   let skipDefaultBuilders = 1;
   let builders = [
     OpBuilder<(ins "TypeRange":$resultTypes, "Value":$cond)>,
+    OpBuilder<(ins "TypeRange":$resultTypes, "Value":$cond,
+      "bool":$addThenBlock, "bool":$addElseBlock)>,
     OpBuilder<(ins "Value":$cond, "bool":$withElseRegion)>,
     OpBuilder<(ins "TypeRange":$resultTypes, "Value":$cond,
       "bool":$withElseRegion)>,
     OpBuilder<(ins "Value":$cond,
       CArg<"function_ref<void(OpBuilder &, Location)>",
            "buildTerminatedBody">:$thenBuilder,
       CArg<"function_ref<void(OpBuilder &, Location)>",
            "nullptr">:$elseBuilder)>,

mlir/lib/Dialect/SCF/IR/SCF.cpp

IfOp::inferReturnTypes(MLIRContext *ctx, std::optional<Location> loc,
   TypeRange types = yieldOp.getOperandTypes();
   inferredReturnTypes.insert(inferredReturnTypes.end(), types.begin(),
                              types.end());
   return success();
 }
 
 void IfOp::build(OpBuilder &builder, OperationState &result,
                  TypeRange resultTypes, Value cond) {
+  return build(builder, result, resultTypes, cond, /*addThenBlock=*/false,
+               /*addElseBlock=*/false);
+}
+
+void IfOp::build(OpBuilder &builder, OperationState &result,
+                 TypeRange resultTypes, Value cond, bool addThenBlock,
+                 bool addElseBlock) {
+  assert((!addElseBlock || addThenBlock) &&
+         "must not create else block w/o then block");
   result.addTypes(resultTypes);
   result.addOperands(cond);
 
-  // Build regions.
+  // Add regions and blocks.
   OpBuilder::InsertionGuard guard(builder);
-  result.addRegion();
-  result.addRegion();
+  Region *thenRegion = result.addRegion();
+  if (addThenBlock)
+    builder.createBlock(thenRegion);
+  Region *elseRegion = result.addRegion();
+  if (addElseBlock)
+    builder.createBlock(elseRegion);
 }
 
 void IfOp::build(OpBuilder &builder, OperationState &result, Value cond,
                  bool withElseRegion) {
   build(builder, result, TypeRange{}, cond, withElseRegion);
 }
 
 void IfOp::build(OpBuilder &builder, OperationState &result,