[SPARK-12873][SQL] Add more comment in HiveTypeCoercion for type widening

I was reading this part of the analyzer code again and got confused by the difference between findWiderTypeForTwo and findTightestCommonTypeOfTwo.

I also simplified WidenSetOperationTypes to make it a lot simpler. The easiest way to review this one is to just read the original code, and the new code. The logic is super simple.

Author: Reynold Xin <rxin@databricks.com>

Closes #10802 from rxin/SPARK-12873.