[SPARK-12771][SQL] Simplify CaseWhen code generation

The generated code for CaseWhen uses a control variable "got" to make sure we do not evaluate more branches once a branch is true. Changing that to generate just simple "if / else" would be slightly more efficient.

This closes #10737.

Author: Reynold Xin <rxin@databricks.com>

Closes #10755 from rxin/SPARK-12771.