[NFCI][SimplifyCFG] Don't pay for a Small{Map,Set}Vector when plain SmallSet will suffice

This *only* changes the cases where we *really* don't care
about the iteration order of the underlying contained,
namely when we will use the values from it to form DTU updates.

GitOrigin-RevId: 1c55dcbca71d2df2fee4564ad53b62505fdbb819
3 files changed