[LoopVectorize] Use SetVector to track uniform uses to prevent non-determinism.

Use SetVector instead of SmallPtrSet to track values with uniform use. Doing this
can help avoid non-determinism caused by iterating over unordered containers.

This bug was found with reverse iteration turning on,
--extra-llvm-cmake-variables="-DLLVM_REVERSE_ITERATION=ON".
Failing LLVM test consecutive-ptr-uniforms.ll .

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D99549