[sml] optimize preprocessing by eliminating unnecessary where function

[winnylyc]

Pull Request

What problem does this PR solve?

Small optimization in sml/preprocessing
Eliminate where function in original solution.
In original version, where was used only for eliminating redundant computation branch, however both branches within the where function still need to be computed, which does not meet the desired outcome.
By eliminating the where function, the computation can be reduced without affecting the result.

[deadlywing]

LGTM