[SPARK-14580][SPARK-14655][SQL] Hive IfCoercion should preserve predicate.

## What changes were proposed in this pull request?

Currently, `HiveTypeCoercion.IfCoercion` removes all predicates whose return-type are null. However, some UDFs need evaluations because they are designed to throw exceptions. This PR fixes that to preserve the predicates. Also, `assert_true` is implemented as Spark SQL function.

**Before**
```
scala> sql("select if(assert_true(false),2,3)").head
res2: org.apache.spark.sql.Row = [3]
```

**After**
```
scala> sql("select if(assert_true(false),2,3)").head
... ASSERT_TRUE ...
```

**Hive**
```
hive> select if(assert_true(false),2,3);
OK
Failed with exception java.io.IOException:org.apache.hadoop.hive.ql.metadata.HiveException: ASSERT_TRUE(): assertion failed.
```

## How was this patch tested?

Pass the Jenkins tests (including a new testcase in `HivePlanTest`)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12340 from dongjoon-hyun/SPARK-14580.