[VPlan] Use SetVector for VPExternalDefs to prevent non-determinism.

Use SetVector instead of SmallPtrSet for external definitions created for VPlan.
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-Unit test VPRecipeTest.dump.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D99544