[SimplifyCFG] Move a portion of an if statement that should already be implied to an assert

Summary: In this code we got to Dom by following the predecessor link of BB. So it stands to reason that BB should also show up as a successor of Dom's terminator right? There isn't a way to have the CFG connect in only one direction is there?

Reviewers: jmolloy, davide, mcrosier

Reviewed By: mcrosier

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D35025

llvm-svn: 307276