[JumpThreading] Prevent non-deterministic use lists

Summary:
Unfolding selects was previously done with the help of a vector
of pointers that was then sorted to be able to remove duplicates.
As this sorting depends on the memory addresses, it was
non-deterministic. A SetVector is used now so that duplicates are
removed without the need of sorting first.

Reviewers: mgrang, efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26450

llvm-svn: 286807