[SQL][Minor] Refactors deeply nested FP style code in BooleanSimplification

This is a follow-up of #4090. The original deeply nested `reduceOption` code is hard to grasp.

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Author: Cheng Lian <lian@databricks.com>

Closes #4091 from liancheng/refactor-boolean-simplification and squashes the following commits:

cd8860b [Cheng Lian] Improves `compareConditions` to handle more subtle cases
1bf3258 [Cheng Lian] Avoids converting predicate sets to lists
e833ca4 [Cheng Lian] Refactors deeply nested FP style code