[MemorySSA] Add all MemoryPhis before filling their values.

Summary:
Add all MemoryPhis in IDF before filling in their incomign values.
Otherwise, a new Phi can be added that needs to become the incoming
value of another Phi.
Test fails the verification in verifyPrevDefInPhis.

Reviewers: george.burgess.iv

Subscribers: jlebar, Prazek, zzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63353

llvm-svn: 363590