[MemorySSA] Use SetVector to avoid nondeterminism.

Summary:
Use a SetVector for DeadBlockSet.
Resolves PR42574.

Reviewers: george.burgess.iv, uabelho, dblaikie

Subscribers: jlebar, Prazek, mgrang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64601

llvm-svn: 365970