### What changes were proposed in this pull request?
Spark org.apache.spark.sql.functions do not have `if` function so conditions are expressed using `when-otherwise` function. However `If` (which is available in SQL) has more efficient code gen. This pr rewrites `when-otherwise` conditions to `If` if it is possible (`when-otherwise` with single branch)
### Why are the changes needed?
It is an optimization enhancement. Here is a simple performance comparison (tested in local mode (with 4 cores)):
```
val df = spark.range(10000000000L).withColumn("x", rand)
val resultA = df.withColumn("r", when($"x" < 0.5, lit(1)).otherwise(lit(0))).agg(sum($"r"))
val resultB = df.withColumn("r", expr("if(x < 0.5, 1, 0)")).agg(sum($"r"))
resultA.collect() // takes 56s to finish
resultB.collect() // takes 30s to finish
```
### Does this PR introduce any user-facing change?
No
### How was this patch tested?
New test is added.
Closes#26294 from davidvrba/spark-28477_rewriteCaseWhenToIf.
Authored-by: davidvrba <vrba.dave@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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