[SPARK-12770][SQL] Implement rules for branch elimination for CaseWhen

The three optimization cases are:

1. If the first branch's condition is a true literal, remove the CaseWhen and use the value from that branch.
2. If a branch's condition is a false or null literal, remove that branch.
3. If only the else branch is left, remove the CaseWhen and use the value from the else branch.

Author: Reynold Xin <rxin@databricks.com>

Closes #10827 from rxin/SPARK-12770.