## What changes were proposed in this pull request?

https://issues.apache.org/jira/projects/SPARK/issues/SPARK-21441

This issue can be reproduced by the following example:

```
val spark = SparkSession
   .builder()
   .appName("smj-codegen")
   .master("local")
   .config("spark.sql.autoBroadcastJoinThreshold", "1")
   .getOrCreate()
val df1 = spark.createDataFrame(Seq((1, 1), (2, 2), (3, 3))).toDF("key", "int")
val df2 = spark.createDataFrame(Seq((1, "1"), (2, "2"), (3, "3"))).toDF("key", "str")
val df = df1.join(df2, df1("key") === df2("key"))
   .filter("int = 2 or reflect('java.lang.Integer', 'valueOf', str) = 1")
   .select("int")
   df.show()
```

To conclude, the issue happens when:
(1) SortMergeJoin condition contains CodegenFallback expressions.
(2) In PhysicalPlan tree, SortMergeJoin node  is the child of root node, e.g., the Project in above example.

This patch fixes the logic in `CollapseCodegenStages` rule.

## How was this patch tested?
Unit test and manual verification in our cluster.

Author: donnyzone <wellfengzhu@gmail.com>

Closes #18656 from DonnyZone/Fix_SortMergeJoinExec.