Let RexBuilder.ensureType() mechanism take place during Rex conversion.

In DRILL-4372, the logic of ensuring same type is removed since, in some case such as below, undesirable cast function will be added and cause failure.

SELECT * 
FROM T 
WHERE (cast(col1 as timestamp)  - to_timestamp(col2,'YYYY-MM-dd HH:mm:ss') < interval 'X XX:XX:XX' day to second)

The fundamental reason for this behavior roots in Drill-Calcite [1], where SqlNode WHERE is expanded to a new object but is not passed into validation step.
[1] https://github.com/mapr/incubator-calcite/blob/DrillCalcite1.4.0-mapr-1.4.0/core/src/main/java/org/apache/calcite/sql/validate/SqlValidatorImpl.java#L3362