unnecessary to extract column into slices when to constant propagation

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

In the solve of constant propagation, the slices of ExtractColumn will be converted into a map and deduplicated. Therefore, directly converting them into a map is better than converting them into slices.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)