Fix a bug around split expression in whole-stage codegen

Author: Davies Liu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -584,7 +584,7 @@ class CodegenContext {
    * @param expressions the codes to evaluate expressions.
    */
   def splitExpressions(row: String, expressions: Seq[String]): String = {
-    if (row == null) {
+    if (row == null || currentVars != null) {
       // Cannot split these expressions because they are not created from a row object.
       return expressions.mkString("\n")
     }

sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/HashAggregateExec.scala

@@ -603,8 +603,6 @@ case class HashAggregateExec(
 
     // create grouping key
     ctx.currentVars = input
-    // make sure that the generated code will not be splitted as multiple functions
-    ctx.INPUT_ROW = null
     val unsafeRowKeyCode = GenerateUnsafeProjection.createCode(
       ctx, groupingExpressions.map(e => BindReferences.bindReference[Expression](e, child.output)))
     val vectorizedRowKeys = ctx.generateExpressions(

sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/BenchmarkWideTable.scala

@@ -29,7 +29,7 @@ import org.apache.spark.util.Benchmark
  */
 class BenchmarkWideTable extends BenchmarkBase {
 
-  test("project on wide table") {
+  ignore("project on wide table") {
     val N = 1 << 20
     val df = sparkSession.range(N)
     val columns = (0 until 400).map{ i => s"id as id$i"}