[BOLT] Fix non-determinism in shrink wrapping

Summary:
Iterating over SmallPtrSet is non-deterministic. Change it to
SmallSetVector. Similarly, do not sort a vector of ProgramPoint when
computing the dominance frontier, as ProgramPoint uses the pointer value
to determine order. Use a SmallSetVector there too to avoid duplicates
instead of sorting + uniqueing.

(cherry picked from FBD14992085)