[SimplifyCFG] Mark missed Changed to true.

Summary:
DominatorTree is invalid after SimplifyCFG because of a missed `Changed = true` when simplifying a branch condition and removing an edge.
Resolves PR42272.

Reviewers: zhizhouy, manojgupta

Subscribers: jlebar, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65490

llvm-svn: 367596