[MemorySSA] Add additional verification for phis.
Verify that the incoming defs into phis are the last defs from the
respective incoming blocks.
When moving blocks, insertDef must RenameUses. Adding this verification
makes GVNHoist tests fail that uncovered this issue.
Subscribers: jlebar, Prazek, llvm-commits
Differential Revision: https://reviews.llvm.org/D63147