[IPO][SampleContextTracker] Use SmallVector to track context profiles to prevent non-determinism.
Use SmallVector instead of SmallSet to track the context profiles mapped. Doing this
can help avoid non-determinism caused by iterating over unordered containers.
This bug was found with reverse iteration turning on,
Failing LLVM test profile-context-tracker-debug.ll .
Reviewed By: MaskRay, wenlei
Differential Revision: https://reviews.llvm.org/D99547