[MemorySSA] Use GraphDiff info when computing IDF.
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@363879 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm