[MemorySSA] Use GraphDiff info when computing IDF.

Summary:
When computing IDF for insert updates, ensure we use the snapshot CFG offered by GraphDiff.
Caught by D63389.

Reviewers: kuhar, george.burgess.iv

Subscribers: jlebar, Prazek, llvm-commits, Szelethus

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63443

llvm-svn: 363879