[TypePromotion] Use SetVectors instead of PtrSets

Remove the chance of non-deterministic insertion of zexts of the
sources by using a SetVector instead of SmallPtrSet. Do the same for
sinks for consistency and to negate the small issue from possibly
happening. The SafeWrap instructions are now also stored in a
SmallVector. The IRPromoter members of these structures have been
changed to references.

Differential Revision: https://reviews.llvm.org/D72322