[SPARK-35351][SQL][FOLLOWUP] Avoid using loaded variable for LEFT ANTI SMJ code-gen

### What changes were proposed in this pull request?

This is a followup from #32547 (comment), where for LEFT ANTI join, we do not need to depend on `loaded` variable, as in `codegenAnti` we only load `streamedAfter` no more than once (i.e. assign column values from streamed row which are not used in join condition).

### Why are the changes needed?

Avoid unnecessary processing in code-gen (though it's just `boolean $loaded = false;`, and `if (!$loaded) { $loaded = true; }`).

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing unite tests in `ExistenceJoinSuite`.

Closes #32681 from c21/join-followup.

Authored-by: Cheng Su <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: c21

sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala

@@ -681,26 +681,29 @@ case class SortMergeJoinExec(
       val cond = BindReferences.bindReference(
         condition.get, streamedPlan.output ++ bufferedPlan.output).genCode(ctx)
       // Evaluate the columns those used by condition before loop
-      val before =
-        s"""
-           |boolean $loaded = false;
-           |$streamedBefore
-         """.stripMargin
-
-      val loadStreamed =
-        s"""
-           |if (!$loaded) {
-           |  $loaded = true;
-           |  $streamedAfter
-           |}
+      val before = joinType match {
+        case LeftAnti =>
+          // No need to initialize `loaded` variable for Left Anti join.
+          streamedBefore.trim
+        case _ =>
+          s"""
+             |boolean $loaded = false;
+             |$streamedBefore
          """.stripMargin
+      }
 
       val loadStreamedAfterCondition = joinType match {
         case LeftAnti =>
           // No need to evaluate columns not used by condition from streamed side, as for Left Anti
           // join, streamed row with match is not outputted.
           ""
-        case _ => loadStreamed
+        case _ =>
+          s"""
+             |if (!$loaded) {
+             |  $loaded = true;
+             |  $streamedAfter
+             |}
+         """.stripMargin
       }
 
       val loadBufferedAfterCondition = joinType match {
@@ -722,7 +725,7 @@ case class SortMergeJoinExec(
            |$loadStreamedAfterCondition
            |$loadBufferedAfterCondition
          """.stripMargin
-      (before, checking.trim, loadStreamed)
+      (before, checking.trim, streamedAfter.trim)
     } else {
       (evaluateVariables(streamedVars), "", "")
     }